Question on UNet and everything connected with it!?

Question

JavaScript

Hello, I have a question on such a topic as an Internet connection and a server-client connection.
Actually, how is this implemented, do I need to write a separate application for the host machine in order to run it through the controller, as if imitating the host to which they will connect? Or how to be here.
And in general, I'm interested in everything that is connected not with the local host, but with the Internet connection to create an online game. If you know links to tutorials on UNet, and what I described, then please attach it in your answer.
Thank you very much in advance.