I'm afraid I don't follow.
Jellyfin has its own users, it doesn't rely on system users. The first time you visit its gui, you go through a wizard to create the jellyfin admin user.
Linuxserver's docker addons do not run as root. They run as user nobody on LE. But that has nothing to do with the user password used in jellyfin gui.
I looked in the /etc/passwd, which contains basic information about each user, such as the username, user ID (UID), group ID (GID).
I don't follow this part. Are you looking inside the container?
With the addon, you simply install it from the repo, wait a bit for it to download the image and create the container, and then you visit http://serverip:8096 in a browser to go through the wizard.
The uid/gid setting is only needed if you're accessing existing media on a remote share and you need to match the perms. Otherwise you don't need to change anything.