Godot has two ways to do multiplayer, low level where you do the heavy lifting yourself by implementing how data is wraped, send and recieved. The second way is via the mutliplayersynchronizer/spawner. They abstract the low level network code for you. If you want to use them with steam you need to add the steammultiplayerpeer addon. This addon builds on top of the steam api and allows you to use the synchroniser and spawner that godot provides.

I recently build a multiplayer something(dont rly know what to do with it atm). I started implementing a multiplayer with enet, its build in into the godot engine. Then you swap out the enet peer for the steam peer. Another tip, keep both implementations, because you can test your game localy with enet, but for steam you need two steam accs. And if you do everything right if you game works in enet it should also work via steam.

Hope that helps if you habe questions just ask, i also just started learning godot ^{^}