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How do I make some maps single player (with a copy of the same boss) and some maps multiplayer?

Hey all,
I was having trouble googling this issue and was wondering.
How do you make a game with single player AND multiplayer mode?
I want to create a game somewhat similar to Pokémon.
Each single player has their own savefile/world (including their own copies of the same enemies etc.)
I would like to create a way to create a shared room for players to walk around in and then challenge each other to battle each other.

How do I designate some maps to be single-player (each having a copy of the same boss) and some maps to be multiplayer?

Thanks for your help.

P.S. While I have easily found content on how to create an entirely multiplayer game online, I have been having trouble finding out how to create a game that has BOTH single/multiplayer, which is why I am asking here. Thank you.