Add opacity and transition to background image and div

  function scrollPictureChange() {
        var main = document.querySelector(".main"),
          sections = main.querySelectorAll(".section"),
          BG = main.querySelector(".BG"),
          el = document.querySelector(".show"),
          cords,
          index = 0,
          h = window.innerHeight,
          lastIndex = null,
          offset = 0;

        applyBG(0);
        window.addEventListener("scroll", function () {
          scrollY = Math.abs(document.body.getClientRects()[0].top);
          index = Math.floor(scrollY / (h - offset));

          if (index != lastIndex) {
            // on index change
            if (lastIndex != null) {
              applyBG(index);
            }
            lastIndex = index;
          }

          el.innerText = `index : ${index} height : ${h} top : ${scrollY}`;
        });

        function applyBG(index) {
          BG.classList.remove("anim");
          setTimeout(function () {
            BG.style.backgroundImage = `url(${sections[index + 1].getAttribute(
              "BGurl"
            )})`;
            BG.classList.add("anim");
          }, 300);
        }
      }

      window.onload = scrollPictureChange;
      window.onresize = scrollPictureChange;
   

body {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    overflow-x: hidden;
}

.section {
    height: 100vh;
    width: 100%;
    display: flex;
    z-index: 1;
    position: relative;
    background-size: 100% 100% !important;
}

.text {
    margin: auto;
}

.text p {
    font-family: 'Lato';
    font-style: normal;
    font-weight: 500;
    font-size: 18px;
    line-height: 149%;
    color: #263244;
}

.text h1 {
    margin-bottom: 20px;
    font-family: 'Lato';
    font-style: normal;
    font-weight: 700;
    font-size: 50px;
    line-height: 0px;
    color: #FFFFFF;
    margin-bottom: 50px;
}

.text .story-detail {
    width: 300px;
    border-radius: 20px;
    background: radial-gradient(76.31% 191.89% at 13.43% 22.19%, rgba(226, 228, 231, 0.8) 0%, rgba(228, 228, 229, 0.368) 100%);
    backdrop-filter: blur(10px);
    padding: 23px;
}

.text .story-description {
    width: 321px;
    border-radius: 20px;
    background: radial-gradient(76.31% 191.89% at 13.43% 22.19%, rgba(226, 228, 231, 0.8) 0%, rgba(228, 228, 229, 0.368) 100%);
    backdrop-filter: blur(10px);
    padding: 23px;

}

.BG {
    position: fixed;
    z-index: 0;
    opacity: 1;
    transition: opacity 10s ease-in-out;
    height: 100%;
}

.anim {
    opacity: 1;
}

.show {
    color: orange;
}

  <div class="main">
      <div class="section BG">
        <div class="show"></div> 
      </div>
      <div
        class="section"
        BGurl="https://i.postimg.cc/9QYL3ytR/mobile-camp.png"
      >
        <div class="text">
          <div style="margin-inline: 20px">
            <h1>Our Story</h1>
            <div class="story-detail">
              <p>
                We saw a gap between what people need and what banks offer. It
                means millions of us aren't getting the banking experience we
                deserve for different reasons.
              </p>
            </div>
          </div>
        </div>
      </div>

      <div
        class="section"
        BGurl="https://i.postimg.cc/9QYL3ytR/mobile-camp.png"
      >
        <div class="text">
          <div style="margin-inline: 20px">
            <div class="story-description">
              <p>
                Traditional banks don’t focus on customers' experience, their
                systems may be slow and outdated, they may prioritize a specific
                group of people, or perhaps they lack the ability to innovate,
                and so on.
              </p>
            </div>
          </div>
        </div>
      </div>

      <div
        class="section"
        BGurl="https://i.postimg.cc/cLPLS8xW/mobile-desert.png"
      >
        <div class="text">
          <div style="margin-inline: 20px">
            <div class="story-description">
              <p>
                So since we're passionate about solving problems and bridging
                gaps, we looked into and identified the challenges and
                capabilities we'll need to build a bank here in the Kingdom.
              </p>
            </div>
          </div>
        </div>
      </div>

      <div
        class="section"
        BGurl="https://i.postimg.cc/mZnqV38T/mobile-birds.png"
      >
        <div class="text">
          <div style="margin-inline: 20px">
            <div class="story-description">
              <p>
                With the best local and international expertise, we began
                building an innovative digital bank designed by and for the
                people. We believe that the most effective way to build a bank
                for people is to do it with them. This is our philosophy. So, we
                started building it with the help of people like you.
              </p>
            </div>
          </div>
        </div>
      </div>
      <div class="section" BGurl="https://i.postimg.cc/k513m0Fb/mountain.png">
        <div class="text">
          <div style="margin-inline: 20px">
            <div class="story-description">
              <p>
                At D360, innovation starts with someone’s passion for improving
                financial services. To that person, we say: Never stop offering
                solutions to your needs. These solutions will not only benefit
                you, but will significantly impact the lives of millions.
              </p>
            </div>
          </div>
        </div>
      </div>
    </div>

const bgImgs = ['https://picsum.photos/id/237/1920/1080','https://picsum.photos/id/102/1920/1080','https://picsum.photos/id/202/1920/1080','https://picsum.photos/id/99/1920/1080','https://picsum.photos/id/63/1920/1080','https://picsum.photos/id/6/1920/1080','https://picsum.photos/id/10/1920/1080','https://picsum.photos/id/19/1920/1080','https://picsum.photos/id/80/1920/1080','https://picsum.photos/id/111/1920/1080'];
function handleScroll(){
    const divs = document.querySelectorAll('.div');
    for (var i = 0; i < divs.length; i++) {
        const div = divs[i];
        const rect = div.getBoundingClientRect();
        if (rect.top >= 0 && rect.bottom <= window.innerHeight) {
            div.style.opacity = 1;
            document.body.style.backgroundImage = `url('${bgImgs[i]}')`;
        } else if (rect.top < 0 && rect.bottom > 0) {
            div.style.opacity = rect.bottom / window.innerHeight;
        } else if (rect.top > 0 && rect.top < window.innerHeight) {
            div.style.opacity = 1 - (rect.top / window.innerHeight);
        }
    }
};
window.addEventListener('scroll', handleScroll);
handleScroll();

html, body {
    /* OPTIONAL STYLE  */
    margin: 20px 0 20px 0;
    padding: 0;
    transition: background-image 1s;
    
    /* The fixed background image */
    background-repeat: no-repeat;
    background-attachment: fixed;
}
.div {
    /* This must be exist. */
    opacity: 0;

    /* OPTIONAL STYLE  */
    width: 300px;
    border:1px solid black;
    border-radius: 20px;
    margin:auto;
    padding: 10px;
    background-color: white;
    transition: opacity 1s;
    margin-bottom: 20px;
}

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Opacity Divs</title>
</head>
<body>
    <div class="div">
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
    </div>
    <div class="div">
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
    </div>
    <div class="div">
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
    </div>
    <div class="div">
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
    </div>
    <div class="div">
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
    </div>
    <div class="div">
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
    </div>
    <div class="div">
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
    </div>
    <div class="div">
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
    </div>
    <div class="div">
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
    </div>
    <div class="div">
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
        <p>Traditional banks don’t focus on customers' experience, their systems may be slow and outdated, they may prioritize a specific group of people, or perhaps they lack the ability to innovate, and so on.</p>
        <p>We saw a gap between what people need and what banks offer. It means millions of us aren't getting the banking experience we deserve for different reasons.</p>
    </div>
</body>
</html>