If you're trying to apply the same policies across a number of relays on the same box, --defaults-torrc FILE can be used to provide default settings, and then each torrc can contain only the unique settings.
But on some OSs / distributions (like Debian), the defaults torrc file is in /usr/share/tor, not /etc/tor.
So if you want to modify it permanently, you'll need to protect that file from your package manager (using something like dpkg-divert), or change the --defaults-torrc path in your service manager (and if you're on Debian, that file is in /lib/systemd/system, and so you'll need to do dpkg-divert on it).
Instead of modifying the shipped unit file + dpkg-* you can also replace specific unit file settings using the so called drop-in files:
systemd.unit man page (debian stable):
Along with a unit file foo.service, a directory foo.service.d/ may exist. All files with the suffix ".conf" from this directory will be parsed after the file itself is parsed. This is useful to alter or add configuration settings to a unit, without having to modify their unit files. Make sure that the file that is included has the appropriate section headers before any directive.
https://www.freedesktop.org/software/systemd/man/systemd.unit.html https://wiki.archlinux.org/index.php/Systemd#Drop-in_files