The default location is: /var/packages/PlexMediaServer/shares/PlexMediaServer/AppData/Plex Media Server/Plug-in Support/Databases.
That is the path that appears in Settings → Scheduled Tasks.
If using Synology File Station, it appears as /PlexMediaServer/AppData/… (File Station hides some path information).
Navigate to that directory using Synology File Station. You’ll see the database files. The backups have the date appended to the file name.
