What you're talking about is called client-server architecture, simply applied to browser games. It's been around for decades. It's not new or uncharted territory, but it's not very common, mainly due to performance issues.
I don't know any guides, but if you Google "Java client-server tutorial" or "Java socket tutorial," you'll find quite a bit of information.
However, Itch doesn't offer any server services for multiplayer games, so everything related to the server side (including the hardware) is up to you.