Additional note for https://linuxhacks.org/how-to-remove-systemd-services/

[PenelopeFudd]

With newer versions of systemd, typing

• systemd-analyze unit-paths or
• systemd-analyze --user unit-paths will dump the list of directories where systemd looks for unit files.

With older versions, type

• pkg-config systemd --variable=systemdsystemunitpath or
• pkg-config systemd --variable=systemduserunitpath

Also, if you delete a unit file before stopping a service, it can get in a state where you can't seem to delete it.
That persistence is actually a feature; the entry lingers so that you can do systemctl status $your_service after it's gone. Once you've read it, do this:

• systemctl stop $your_service -- if not stopped already
• possibly wait a minute or two for the stop process to time out
• systemctl reset-failed $your_service