The Unix underpinnings, frameworks, and platform security model behind Apple's desktop OS.
Force Quit doesn't work, the app's icon keeps bouncing, or it's stuck at 'Not Responding' indefinitely. Here's the actual escalation path from gentlest to most forceful.
Working through the network preference files and configuration state that macOS updates most commonly leave in an inconsistent state, before assuming a hardware or router problem.
A complete walkthrough 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 application's code signature and notarization are actually valid from the command line, instead of relying only on Gatekeeper's pass/fail dialog with no further detail.
Understanding the container/volume distinction that makes APFS fundamentally different from older partition schemes, and using diskutil directly to add, resize, and manage volumes without wasting disk space on fixed partitions.
A complete walkthrough resetting NVRAM and the System Management Controller — two different low-level resets, solving different categories of problems, on both Apple Silicon and Intel Macs.
Using the same BSD packet filter that powers macOS's Application Firewall directly, for network-layer 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, understanding why an app sometimes doesn't re-ask for a permission it should, and what the TCC database actually is under the hood.
Starting, stopping, inspecting, and restoring from Time Machine backups without touching the GUI, useful for scripting backups or diagnosing why one is stuck.