HTTP - headers, the concept of forward/reverse proxies, etc
TLS - not necessarily the underlying cryptographic primitives but the high-level overview, how certificates are obtained/managed, the data they encode, how are they validated, etc
DNS - the types of records, etc
The best way IMO would be to learn by doing - set up a basic server-side rendered application (you can add a JS frontend later, this will come with its own challenges such as serving it from another domain and setting up CORS, etc) purely on a dedicated server or even self-hosted hardware. It's not that the cloud is always a bad idea (though it can be in some cases), but doing it by hand from scratch will teach you plenty of things that the cloud otherwise abstracts away from you.
Get yourself a Linux VPS from a provider offering fixed, capped pricing (so a mistake can't cause a huge bill) and serve that web app from it, doing everything yourself - no S3, no Cloudflare/AWS ELBs, etc.
You won't learn anything in depth, but you'll get a good high-level overview of how all these concepts fit together and will be able to know how to dig further and where to dig.
Congrats you know networking now.
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.
But I'm not really sure what backend-specific networking knowledge you need.