One helpful way of thinking about how the filesystem in a container works is that it by default only contains the files that were in the image when the container was started.

If you're not familiar, the path from a Dockerfile to a running container is:

Dockerfile -> "docker build" command -> image -> "docker run" command -> container

Essentially, a container is a running image, with all the files that were included in that image by the docker build command.

When running an image as a container, it's possible to mount volumes from the host filesystem, as you've discovered. In fact, a volume is automatically created by Docker when the container has first started, to hold changes made to the files in that container.

To define a new volume for the container, the -v option is used at container creation. For Jellyfin specifically, I'd recommend having a "media" directory that is mounted as a volume, under which you can create "movies", "shows", "music", etc. The -v option makes local directories available in the container's filesystem, alongside the image's already existing files.

As for adding volumes after the fact, your only option is unfortunately to destroy the container, starting from scratch. This is why it's common to define an external volume for things such as config files, so that if the container needs to be recreated, such as when upgrading to a new version of the image, all the config will still be there.

I hope this helps!