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.
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.