[PSA] How to list systemd units and services

Today, I was doing some troubleshooting, and came across this Web Page that was very useful.
It tells how to list active and/or inactive units, list active and/or inactive services, list all the installed systemd services (even if they are not loaded), and list all systemd services that will be run at each boot automatically.

It helped restore a little sanity to my dilemma.