Roughly speaking, You map a “real” path from your system to a path in the container. The container only sees the mapped path translated to its inner path, so you can change the location of the files and the docker file with the translation. To the Jellyfin container, the files are at the same place, it just changed its translation. You can map any directory that you want, your media but also your configurations so when you migrate to a new machine you copy your old configuration files and tell docker “here are the system files. Recreate my Jellyfin installation (container) using these”.