My advice would be to start with web applications and use for example Rust (because it is easier to get started on simple stuff) and build a TCP server (20-30 lines of code).

Then ask yourself what you are seeing and why. OSI model and such. Then try to find out what the kernel has to do to setup a socket and build something in C (just google Linux socket programming).

And after that, try researching how the packets you receive can be blocked by a firewall and why.

And go on and on. I am currently writing a book which touches the topic a bit and it helps to build stuff and see packages.

For example: Use Wireshark and try to find out how you can see the communication of the browser.

Google: - Linux Networking - What are sockets - Socket programming - Firewall blocks a domain how - How to bypass firewall for domain