How do I share a project with a friend?

(Haven’t seen the video yet, but at this point I just want to know how to make things work)

I really don’t need the history of git or anything like this; all I need is just a quick answer, maybe even in 2 words… how do my friend and I work on a game together? What do I or my friend need to do to make things work, so that both of us could work together on the same project and we both could see what’s going on?

Look at this from this point of view… You and your friend are developing… Eh, let’s for simplicity say, Super Mario Bros, the one from NES era. You are responsible for coding and your friend is responsible for design. (This is for example, but it could be the other way around.) As you test the game, you notice that the brick tiles don’t break. All because they’re 1 pixel above the “hitting line”. Or you can’t jump into the pipe because it’s too narrow. These are easy to correct, yes, but so far, you both were developing it independently, and so didn’t have an idea of what was going on. If both of you could see it, if both of you could test it independently from each other, it would have been easier to actually see what was the problem, and configure it accordingly, and in a timely manner. Right? Or am I completely wrong here and none of it matters in the first place?!

This is what I’m trying to figure out, whether my friend needs the project files in the first place, or what he can actually do… I do have a .gitignore file, and it ignores nearly 11.9 gb of files, so… back to example with Mario, from my point of view, without these files, it’s like he can’t see where he should set the hitting line and as such, it’ll make the game unplayable, or at least, not enjoyable… The players can’t hit the brick tiles, they can’t get the mushroom, they can’t jump into the pipes to get the bonus… So it’s no fun, and adds up extra work for both of us. Or am I wrong?