Really? Can you give some examples? Just for multiplayer or for large asset transfer?

Steam has a feature for this where for supported games like Counter Strike 2 or Team Fortress 2, your connection will be routed through other players, or just people on your friends list depending on your settings.

Off the top of my mind as a big game GTA V runs P2P, I think most games that aren't competitive do, only using servers for matchmaking

I was going to respond that most games from the PS3/360 era used p2p but I don't know about recent ones. But GTA5 was a PS3/360 game.

Off the top of my head Warframe is massive and uses p2p while in most missions, Helldivers 2 was a huge launch last year with p2p, dead by daylight, a lot of indie multi-players like risk of rain 2. It's quite popular for coop style games where you need to worry less about cheating. They will use servers to create the matchmaking and then will pick the user with the strongest computer and network typically to actually host the game. I don't know of any games that do p2p asset transfer anymore