Hello, this is a small program that should demonstrate that multiplayer also works in the BGE. Befor one starts the Blender script one must start the server (server.py). When you want to edit this game on your PC edit it in server/data/ not in client/client-data/ otherwise it will deleted when you start the client.
it is possible to update the BGE network Multiplayer Blender 2.7 that it works under UPBGE Blender 2.83
I didn't used UPBGE. But I can recommend the godot game engine. It's own programm language is similar to python. https://godotengine.org/
AFAIK the old BGE which this was written in (very badly as I might add) has been discontinued. There seems to be some continuation but I have no clue how that works. In general networking for multiplayer games is integrated into engines as it is quite complicated to keep that state of the game synchronous between all the clients.
So if the engine you are using does not have such a feature built in, I at least recommend using a library to abstract and serialize the data sent (which I have not done and is really bad for performance).
The real trick for getting this to work in the old BGE was to use attributes which are global variables which were supposed to have types but were not type checked. So you can save objects in there which represent the network connection. I just used plain sockets but as said above you should really use a library, unless you really know what you are doing.
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Im on android right now, but damn, if this is well written, then I just might have to finish that game I gave up recently!