DNS records are usually set once and forget. You read the documentation once when you’re doing it, and then you basically never have to touch it again. I had to look up the difference between a CNAME and an A record, even though I’ve set them numerous times before. Not useful in day-to-day.

DNS servers: going to assume you query a DNS server that has a map of domain names to IP addresses, with basic routing via DNS records, which then resolve to a final address which is sent back. Again, never going to have to know this unless I’m a network admin.

Port-mapping: what is there to understand? I have literally never had to map ports unless I’m port-forwarding a game server I’m running on my local machine, that’s using a router to be exposed to the outside network. Literally, every cloud provider will give you an endpoint address that does not require port-mapping. If you’re setting up a company network you have to port-map, but not if you’re deploying a simple server/app. Ergo, network admin’s job.

Compilers: lol. There’s tons of languages, and tons of different compilers for each language. Then there’s distinguishing compilers, interpreters, compiler-interpreters, and transpilers. At the end of the day they take text (usually) and transform it into another form. I’ve built a compiler before (toy AST & recursive descent), but I have never needed to know about it in a CRUD context.