1 year ago , I couldn't answer to your question ... But now I can !
In Theory , the multiplayer is just 1 server where people , clients , can connect ... the two of them send themselves messages each tick , that's all ! Practically , To make a server , you just need to host it on a Pc ( Open the Server App on that PC )
You can use Zack0Wack0 websocket server ( My Bad , I meant plugin ) for the client , and a Node.Js as a server !
Cheers !