How to convert this game into a multiplayer game?

I have been watching a youtube tutorial series.

I haven’t finished it, but I have reached a point where my code looks like a game. However this tutorial series doesn’t cover multiplayer elements, so I tried and tried to see if I could find any other multiplayer java game tutorials online, but there were none. So can anyone simply take this code and add a GameServer, GameClient class to it so that I can have a server running which transfers the player’s co-ordinates and updates them to all clients? In theory it looks simple, but I have been trying it for over a week and I can’t figure out what to do. I have attached my game source code here. Would be a great help if someone could do this?