Understanding socket units
The socket unit files are also in the /lib/systemd/system/ directory, and their filenames end with .socket. Here's a partial list of them on one of my Ubuntu Server machines:
donnie@ubuntu20-10:/lib/systemd/system$ ls -l *.socket -rw-r--r-- 1 root root 246 Jun 1 2020 apport-forward.socket -rw-r--r-- 1 root root 102 Sep 10 2020 dbus.socket -rw-r--r-- 1 root root 248 May 30 2020 dm-event.socket -rw-r--r-- 1 root root 197 Sep 16 16:52 docker.socket -rw-r--r-- 1 root root 175 Feb 26 2020 iscsid.socket -rw-r--r-- 1 root root 239 May 30 2020 lvm2-lvmpolld.socket -rw-r--r-- 1 root root 186 Sep 11 2020 multipathd.socket -rw-r--r-- 1 root root 281 Feb 2 08:21 snapd.socket -rw-r--r-- 1 root root 216 Jun 7 2020 ssh.socket . . . . . . -rw-r--r-- 1 root root...