For a web frontend dev switching to backend I'd assume you still want to stay within the web space rather than let's say systems programming - in this case I wouldn't worry too much about the lower-level details yet and instead focus on what's relevant for the web:
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.
I would still learn at least basics of TCP and IP, so that you are aware e.g. what firewalls do and how this shows up on the client side (vs when nothing is listening on the port but the port is otherwise open), what private IP ranges are, or what a timeout look like. These are the basics which are going to be extremely relevant when you start troubleshooting. Especially if they slap a docker image on their VPS. There's a frightening number of people who can't tell DNS resolution failure from a timeout, think they are going to get HTTP 500 if server breaks the connection, or don't take those failures into account (and it shows up in how they design their code).
The W Richard Stevens book "TCP/IP Illustrated, Volume 1" taught me, but it's not really a good overview: more of a deep dive.
Still, that along with his companion volume "Unix Network Programming, Volume 1" will help set you up for a rewarding career.
A quick search suggests both are available as PDFs from multiple sources, or you can get the hardcover Bible-style editions, which I personally find quite fitting.
Thanks a lot for your answer!
I agree on the thing that doing something would actually give me knowledge for that. But still if you can share some resources then I would be very grateful to you.
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.