I've actually been super curious lately what happens if you route a PC's localhost to someone else's IP, but I have no clue how to ask on SO without baiting out

I've actually been super curious lately what happens if you route a PC's localhost to someone else's IP, but I have no clue how to ask on SO without baiting out the obvious "don't do this" and the generic "stuff won't work". Obviously, I could do it myself and explicitly see stuff not working, but I'd like to find out what actually happens under the hood and why things break; SO seems like an awful place for these kinds of "what if" questions and I don't know a better place to ask.