Why is there a green dot popping up when I use Snapchat in the first place ?

That is an indication that the camera is in use. See this, About the orange and green indicators in your iPhone status bar - Apple Support

But why is that even there in the first place ?

Because that is the way that Apple designed iOS 14. It is there to tell you that your camera is active.

Why are they just now putting a green dot ?

Only Apple could answer that question, and we cannot speculate here, it is a violation of the Terms of Use.

It is what it is.