This is a very common question, I'm sure you can find many articles or videos about "how do I start" when getting into game dev with an engine. As you already have a development background the best way for you is probably to "just build something" using the official documentation only. Pick a "small" game or project as a way to work with the engine and learn how things are done and how all the pieces interact with each other. The more you understand all the bits and pieces and how they are composed together to form a game the easier you will find it to plan and conceptualize your game.

Multiplayer is a super interesting topic, especially regarding these mysterious "game servers" or online services. I recommend Nakama as a FOSS example for one such service - which conveniently also has Godot support. The example project of theirs might also be a good starting point for you to see the engine at work.

Alternatively you can also use your backend experience to build the game services yourself - but that will distract you easily from working on the game itself.