The Unix underpinnings, frameworks, and platform security model behind Apple's desktop OS.
Force Quit doesn't work, the icon keeps bouncing, or it's stuck 'Not Responding' indefinitely — the actual escalation path, gentlest to most forceful.
Working through the network preference files macOS updates most commonly leave inconsistent, before assuming a hardware or router problem.
Creating, listing, mounting, and cleaning up APFS local snapshots directly — the same mechanism Time Machine uses, available for manual, ad-hoc use.
Checking whether an app's code signature and notarization are actually valid from the command line, not just Gatekeeper's pass/fail dialog.
Understanding the container/volume distinction that makes APFS different from older partition schemes, and using diskutil to manage volumes directly.
Resetting NVRAM and the System Management Controller — two different low-level resets solving different problems, on Apple Silicon and Intel Macs.
Using the same BSD packet filter that powers macOS's Application Firewall directly, for filtering the GUI's simple allow/deny-per-app model can't express.
Turning on macOS's built-in SSH server correctly, then hardening it beyond the default configuration with key-based authentication and restricted access.
Resetting stuck permission prompts, why an app sometimes doesn't re-ask for a permission it should, and what the TCC database actually is.
Starting, stopping, inspecting, and restoring Time Machine backups without the GUI — useful for scripting backups or diagnosing a stuck one.