For experienced sysadmins: what OS fundamentals should a system administrator actually understand?

I’m looking for the big-picture OS knowledge that is genuinely useful for understanding how systems communicate and operate across the network.

For example: processes/threads, memory management, filesystems, permissions, system calls, sockets, TCP/IP interaction, services/daemons, kernel vs user space, I/O, DNS, routing, logging, etc.

I’m not asking for a list of commands or vendor-specific administration skills. I’m trying to understand which OS concepts are fundamental enough that a sysadmin should understand how they work under the hood, and how those concepts connect to data communication between machines.

What would you consider the essential OS fundamentals for building that overall mental model?