unity-game-engine shader alpha rgba shader-graph

How does an image'alpha transfer so much information to other nodes in Unity Shadergraph?

I have a image:

The upper part of this image, which alpha value is 1 (or 255 in RGBA)

The lower part of this image, which alpha value is 0.3, I used it for shadow in game.

So When I import it to Unity ShaderGraph as a _MainTex, when I split it alpha, it looks like this:

My first questions is:

"alpha" is actually a VECTOR 1 type in Unity Documention, but as I could see from the preview, there are three colors, black indicates alpha's value 0, hard white for alpha's value 1 and soft white for alpha's value 0.3, how can one single value transfer so much messages?

My first understanding is:

each pixel's alpha value is stored in the images already, the "alpha" in the shadergraph is just

like a global parameter to control them based every pixel.[I dont know if this is correct]

but when I give alpha a smoothstep node, I

am going to set the pixels's alpha under 0.3 to 0, I found it worked like this:

smoothstep added to the alpha, as you can see, 0.3<0.99, so

the translucent of the image is removed!

So here comes my second question:

Since "alpha" in the input works like a global parameter, how does it affect a picture separately?

My second understanding is:

"alpha" is just like an one-dimensional array, it stores transparency likes this:

{1,1,1,0.3,0.3,0.3}

and when it calculated by smoothstep,its value will be changed like this:

{1,1,1,0,0,0}

But it comes to my first question, ALPHA IS A VECTOR1 TYPE, it only has one value to edit