systemd is the init system on virtually every modern Linux distro, and it does far more than start daemons — timers, mounts, path watchers, socket activation, sandboxing. The exact directive is easy to forget between uses, so this site collects the units that actually come up into one library.
Each recipe is a complete unit file with a short explanation, the daemon-reload / enable --now commands, verification, and the gotchas. Recipes target modern systemd (v240+); newer directives are flagged.