Linux units
Units are configuration files that describe the resources that are managed by systemd. They are stored in four filesystem paths. If I do a manual search I can approximately tell where they are:
Wassim@linux:~$ find / -mindepth 1 -name systemd 2>/dev/null
/home/Wassim/Documents/ddupdate/systemd
/home/Wassim/.config/systemd
/run/systemd
/run/udev/tags/systemd
/run/user/1000/systemd
/etc/xdg/systemd
/etc/systemd
/var/lib/systemd
/var/lib/flatpak/runtime/org.gnome.Platform/x86_64/49/e51263e53d04900556e2f97ac2b27201f632f604d68d67f50323f9a99389fdb0/files/lib/systemd
/var/lib/flatpak/runtime/org.gnome.Platform/x86_64/49/e51263e53d04900556e2f97ac2b27201f632f604d68d67f50323f9a99389fdb0/files/share/licenses/gnome/systemd
/var/lib/flatpak/runtime/org.gnome.Platform/x86_64/50/a8da766dd0273a67539d2b98358ed6809d8e729280baf63428108837135229d3/files/lib/systemd
<---- output omitted ---->
/var/lib/snapd/snap/gnome-3-38-2004/143/usr/lib/systemd
/usr/lib/systemd
/usr/lib/systemd/systemd
/usr/lib64/systemd
/usr/lib64/python3.12/site-packages/systemd
/usr/share/licenses/systemd
/usr/share/doc/systemd
/usr/share/systemd
/usr/share/cockpit/systemd
Wassim@linux:~$
/run/systemd seem to store runtime unit files, because /run/ typically contains runtime data, so it can not be a permanent location of systemd unit files. There is also /etc/systemd and /usr/lib/systemd/system/. The latter appears to store system units: