Do you have any recommendations for learning multiplayer development?

Hello everyone,
I am currently able to do the following with Unity:
A first-person game with a character and a simple setting (Character and Scene)
An easy 2D game (with a character and a scene)
Making a user interface
How does C# work? Does it have a programming language?
It is multiplayer games (I mean online multiplayer) that interest me the most.
It will motivate me to learn more if that happens.
So what basics do you think, I should have before tackling that? And especially what tutorials/documentations could you recommend?
What are the alternatives to photon? It's hard for me to know where to begin.
I appreciate your help.