Writing your own client-server chat application. How?

Question

Windows

Good afternoon.
In my spare time, I decided to delve into network programming and understand how everyone's favorite chats are written. Which are full on the Internet in the form of source codes, but nowhere is there a good explanation.
The first thing that interests me is writing a server part that could successfully work on a Linux or Windows server.
I have been digging on YouTube for a long time, looking at examples of ready-made chats, but in most cases people simply do not understand why and what they write. Having written off a ready-made example from somewhere, only tweaking it a little.
I would like to study this issue in more depth. So that on occasion I would know which part of the code is responsible for what.
Maybe there are some books on this subject. Or good articles on the Internet that I might have simply overlooked.
C++ or C# language. If it matters.

Answer 1

Open the wonderful book by Jon Sneijder "Effective tcp / ip programming" and read.