I am a college student. I have now used cocos2d-x to create offline games. In the future, I want my offline games to be able to play online with many other players through an internet connection and a server-client model. What additional knowledge do I need to learn to be able to do so? Please teach me. You can give me keywords, links or anything that can help me. I am so grateful! In addition, I also have many questions looking forward to answer: -I want to be a game developer for mobile(online game). Should I learn how to use engine games or should I not use them? -I learned through C ++ and am learning OpenGL. After I finish OpenGL, what do I need to learn to be able to make offline games for mobile?

I tried searching with Google but I still don't know what I need.
(I know it will take a lot of time and effort but please show me how to go!)

Well, at minimum you need some sort of server connected to some sort of database, and your client needs to talk to it over some sort of network protocol.

The details, however, vary an awful lot based on the type of game.

For a turn-based or asynchronous game (i.e. things like Civilisation or Words with Friends) you can use pretty much any web server language/toolkit (say, Node.js), pretty much any database (say, Firebase), and the client can talk to the server over a standard web protocol like HTTP.

For real-time games things are a little more complicated, and you generally can't just use a website as the backend. There are off-the-shelf toolkits for online games that can take a significantly chunk out of the learning curve (say, Photon). Otherwise you get to learn about TCP vs UDP, state synchronisation, client-side prediction, etc.