systemd

systemd services and units

journalctl

Tested journalctl examples: filtering by unit, priority and time, following logs live, output formats, and controlling journal disk usage.
systemctl

Tested systemctl examples: starting and stopping services, enabling them at boot, reading unit state, drop-in overrides, and diagnosing failures.
cron vs systemd timers

Debian ships both and uses both. What differs is where the output goes, what happens to a missed run, and whether you can test the schedule.
Managing services with systemd

How Debian ships, enables and starts systemd services, where unit files live, and how to override a packaged unit without ever editing it.