>>329770205
You join a lobby with other players. Everyone can suggest songs and the host picks the song to play. Then you play the same song at the same time so it's synchronized and you can see what the others are doing.

The extra mods are needed to make the base game multiplayer work with custom songs. If you've never touched custom songs I recommend trying them out strongly. Game is much more fun with more than the base game music. Google "beat saber custom songs" to get started.