Not the OP here.
If someone is writing a new program, do they really need to know how the GET request works at the level you post, and the difference between header and body?
I think this is actually quite important to know "enough" about. Should I expect you to know all the headers out there? No. I also don't know everything that's around nowadays. Should you I be able to expect that you know the basics of how a request works, the fact that there are headers and bodies and query params and what's different between get and post in those regards? Oh absolutely. Like the OP says, fundamentals. If you know the fundamentals of the split between headers and body you can dig into the specific ones when needed. Do I remember the exact way content transfer encoding stuff works w/ chunking in all details and edge cases? No. But I have enough fundamental understanding of how HTTP works that I can understand when I google it.
also expect the candidate to know deeper network levels? How a TCP header is set up? An IP header? Ethernet?
I think it's important to understand the basics of networking unless you're literally someone that just does CSS. You should have a basic understanding of a TCP connection. Do you need to be able to tell me anything about SYN/SYN-ACK/ACK? No. Should you know that DNS exists to turn the names we all use into IP addresses and that computers actually create connections between systems using those IP addresses and ports? Absolutely!
Do I expect that someone from a bootcamp knows any of this? No, but that's also why I don't like even considering bootcamp people. Been there, burned by that.
For a backend person, even if entry level? Oh yes you better know some TCP/IP. Do you need to be able to recite what a TCP or IP header look like from memory? Of course not, I couldn't tell you either any more. Should you be able to tell me that TCP is implemented on top of IP and you can use one without the other? Maybe not for an entry level person.
What if they have no idea, but can tell you if they wanted that info, they'd look it up on MDN, or pull up a browser's network tab or
Wireshark? If someone tells me they know what wireshark is, they're basically hired (yeah sure I'd also still make sure they didn't just pick up the word and know their TCP after that but I bet you the signal to noise ratio on that is pretty good.