How can I learn how to develop multiplayer games?

Hello everyone!
The following are some of the things I can do with Unity:
(Character and Scene) Featuring a character and a simple setting
There is a character and a scene in this easy 2D game
Interface design
How does C# work? Does it use a programming language?
My favorite games are multiplayer games (online multiplayer).
In such a case, my motivation for learning more will be sparked.
Would you mind telling me what basics I should have before I begin? Would you be able to recommend any tutorials or documentation?
Is there an alternative to photon? I don't know where to begin.
It is greatly appreciated that you helped me.

 

YouTube is your friend.

 

Does multiplayer have lag? I often play online multiplayer games and feel very laggy, is there any way to fix this?

 

If you're playing online, there is one solution but it won't be cheap, get faster Internet