Where can I learn how to develop high-quality server-client applications?

Question

C++ / C#

I'm trying to figure it out and create my own server-client application, I work with the TCP protocol, data is transmitted, processed, received, etc.
I want to know a lot of questions about how to do them right so that 1 connection goes on and work on it until it breaks, which I can't implement. You have to make a connection, send data that this is a connection to the server, then cut off the connection (why? why?), etc. I am working on a messenger, then each message sent by a person is his connection to the server (again? for what?), sending a message, cutting off the connection.
Also, there is no way for me to check whether the person disconnected from the server, that is, did he close the client application? After all, he connects - sends - goes into the fog (Shutdown, Close).
I would like to ask you and ask you to share information about resources or books that describe this process in order to read it once and understand how to work with Internet protocols.
Thank you very much in advance. Below I will attach links to the code of my terrible server-client. [Connect to server]
Server: pastebin.com/2W8ftJia
Client: pastebin.com/bsJwrAQD

Answer 1

Answers.
en.stackoverflow.com/questions/635000/%D0%9A%D0%B0...

Answer 2

Write me a PM: vk.com/id99551920, I'll tell you how to arrange everything correctly and beautifully