Just search for TCP handshake on Youtube.

Then install a program called Wireshark. Search Youtube on how to use it to inspect traffic.

Then read this excellent article on how web frameworks actually work: https://shopify.engineering/building-web-app-ruby-rails

Unless you're looking to build your own web framework or something that's all you need to know for backend development because everything is nicely encapsulated these days.

If you want to build frameworks and stuff then learn C and grab a book called TCP/IP Illustrated.