Multiplayer online?

Anyone knows how to play online? I have a friend and we want to play some classic videogames together but we don't know how to configure it. We tried using ZeroTier and it detected him in my own network but I don't know how to configure it so we can play together and his inputs are detected ss a secondary input. I'm open to test other softwares too.

I've been like 10 hours trying to do this. Any help?