The `RuntimeDirectory=` directive does not create a new mount, it only creates a new directory under the existing `/run`. So, in a way, you're just reusing space from the existing `/run`.
In other words, look for `/run` in `mount -l` output to see the options of the mountpoint where `/run/plex` lives.
You can also use the `findmnt(8)` command and pass it the full path with `-T` to show where the mount point is and its options. For example:
$ findmnt -T /run/plex
TARGET SOURCE FSTYPE OPTIONS
/run tmpfs tmpfs rw,nosuid,nodev,seclabel,mode=755
If you want to know how much space has been allocated for the in-memory tmpfs, you can use the `df(1)` command:
$ df -h /run/plex
Filesystem Size Used Avail Use% Mounted on
tmpfs 3.9G 612K 3.9G 1% /run