This is such an awesome post for who wanna start multiplayer game. Recently I also start creating multiplayer and socket.io really helpful in send and receive data in order to achieve real time method. My game is only for two player per session but that is impossible that only client able to join game at the time. I trying to create an custom room like what u did in MiniRoyale.io (create custom session) so they able to join room in another space.
it will automatically create a random link and your friend able to join through this link. I try to research on this function and already stuck here for whole month. I try using socket.io namespace and join room function but the output not that I expected and I think I lack some kind of element.
If u see this post can reply me what the tech slack that u use to create this random link custom session or any tip that I able to explore it. Thank you so much if u able to reply. Btw I love this game 
Thanks for create this kind of game that is fun and addictive.