How does switching out standard frames work in general?

[Chalybeum]

Hey there, just signed up and for now I have just a little question for my general understanding.

Let's say I want to write an addon that replaces some UI element, like the vendor interface. Is my new frame loaded after the standard ones and therefore automatically replacing the old one? That would imply I simply name the frame the same as the standard one.
Or do I simply hide the original and display my version. That would mean both would be processed which doesn't seem to be desirable as far as I can tell.

I am aware, that this is a very broad question but it should only serve to broaden my own understanding. I hope some of you can shed some light on this one. 'Till then, have a nice day.