Where to read about the "combat" use of sockets?

Question

Computer networks

Good afternoon.
Somehow the topic of working with sockets bypassed me (well, I didn’t come across tasks in which, in practice, one could try to work with them). I decided to fill this gap in knowledge.
It is clear that I read articles, looked at implementation examples, but all this is at the level of academic examples "from a textbook". I would like to know more about the use in real "combat operations".
For example: Let's say there is a chat (or an online game, or some other system of constant information exchange between users). I wonder if each socket keeps a permanent connection to the server? If it holds, then how then does the server not go to bed from 5-10 thousand online? And if it doesn’t, then how do clients determine that a message has arrived in the chat and that it needs to be received and displayed?
Or, for example, what are the pitfalls with buffers. If I have a limited buffer size for receiving a message and someone sends a longer message, then what happens to the cut data on the receiving side? And what is the best way to handle such situations?
In general, give something to read on similar topics, please. (Well, or at least answer these two questions :))
Thanks in advance to all who answered.

Answer 1

Jon Sneijder - Using TCP/IP Effectively