Why can "Class 'DI\Bridge\Slim\App'" not be found? PHP

I am trying to create a shopping cart for my website, I am following the tutorial below: https://www.youtube.com/watch?v=tRh467FX12U&list=PLfdtiltiRHWH9JN1NBpJRFUhN96KBfPmd&index=2

In the tutorial, he uses 3 different imports from GitHub and this is how installed them in the terminal using composer:

rosscurrie = Name of ~user

@Ross-Air = Name of Macbook

MobileMastersNew = Name of the folder that holds all website files

composer = is installed globally

require <...> = the imports from GitHub

1. rosscurrie@Rosss-Air MobileMastersNew % composer require slim/slim:^4.0
2. rosscurrie@Rosss-Air MobileMastersNew % composer require slim/twig-view:^3.0
3. rosscurrie@Rosss-Air MobileMastersNew % composer require php-di/slim-bridge
4. rosscurrie@Rosss-Air MobileMastersNew % composer require illuminate/database

I have limited experience with Laravel but not completely unfamiliar. When I try to load the index.php page it gets this error:

Fatal error: Uncaught Error: Class 'DI\Bridge\Slim\App' not found in /Users/rosscurrie/Sites/MobileMastersNew/app/App.php:8 Stack trace: #0 /Users/rosscurrie/Sites/MobileMastersNew/vendor/composer/ClassLoader.php(444): include() #1 /Users/rosscurrie/Sites/MobileMastersNew/vendor/composer/ClassLoader.php(322): Composer\Autoload\includeFile('/Users/rosscurr...') #2 [internal function]: Composer\Autoload\ClassLoader->loadClass('Cart\\App') #3 /Users/rosscurrie/Sites/MobileMastersNew/bootstrap/app.php(9): spl_autoload_call('Cart\\App') #4 /Users/rosscurrie/Sites/MobileMastersNew/public/index.php(3): require('/Users/rosscurr...') #5 {main} thrown in /Users/rosscurrie/Sites/MobileMastersNew/app/App.php on line 8

My folder directory is as follows:

My ../MobileMasters/app/App.php is:

<?php

namespace Cart;

use DI\ContainerBuilder;
use DI\Bridge\Slim\App as DIBridge;

class App extends DIBridge
{
    protected function configureContainer(ContainerBuilder $builder)
    {
        $builder->addDefinitions([
           'settings.displayErrorDetails' => true,
        ]);

        //
    }
}

My ../MobileMasters/bootstrap/app.php is:

<?php

session_start();

use Cart\App;

require __DIR__ . '/../vendor/autoload.php';

$app  = new App;

My ../MobileMasters/public/.htaccess file is:

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ index.php [QSA,L]

My ../MobileMasters/public/index.php is:

<?php

require __DIR__ . '/../bootstrap/app.php';

$app->run();

My ../MobileMasters/vendor/autoload.php is:

<?php

// autoload.php @generated by Composer

require_once __DIR__ . '/composer/autoload_real.php';

return ComposerAutoloaderInit2e9ebb4be0f95ed0dbfb26486b8ba4b7::getLoader();

Lastly, my ../MobileMasters/composer.json is:

{
    "require": {
        "slim/slim": "^4.0",
        "slim/twig-view": "^3.0",
        "php-di/slim-bridge": "^3.0",
        "illuminate/database": "^7.2"
    },
    "autoload": {
        "psr-4": {
            "Cart\\": "app"
        }
    }
}

You should use use statement after autoloading:

<?php

namespace Cart;

use DI\ContainerBuilder;
use DI\Bridge\Slim\App as DIBridge;

class App extends DIBridge
{
    protected function configureContainer(ContainerBuilder $builder)
    {
        $builder->addDefinitions([
           'settings.displayErrorDetails' => true,
        ]);

        //
    }
}

And change this file also:

session_start();

require __DIR__ . '/../vendor/autoload.php';
use Cart\App;

$app  = new App;