Sorry, saw this just now.
Maybe PC players and Steamdeck users can't be in the same lobby, but I'm not sure about that.
Sometimes the game put players from the same lobby in separate sessions due to geographical distance and other reasons. In that case players can still join each others sessions if they are friends in Rockstar Social club - you just open your Social club from your in-game menu, find this friend and join his/her game - you will be moved to his/her session.