How do I share a project with a friend?

Guess what, that’s exactly what I’m trying to figure out! How to make this work for both of us! Like, if only I can “see” what’s going on, then what’s the reason to make it available to work on the game with others in the first place?! So, I need to find a way to transfer the project to my friend, and do it the way that it’ll work on his computer, as well as on my computer, and so that he could also see what he’s doing/working on.

I guess, what I’m trying to ask is… how does revision/source control work in the first place? If, say, I’ll be responsible for the story/some design, and my friend will be responsible for coding, how would that work? Does he just code, and I test his coding? Or how “exactly” does it work?

About Source control/revision control - in UE it’s called Revision control… I know it’s called source control, but… blame Epic Games for calling it Revision control…