> you are running jellyfin on a device, like a NAS. it reports telemetry

Jellyfin Server packages are hosted on repo.jellyfin.org, so the Jellyfin organisation gets the data that you're probably still running Jellyfin and want updates, not Microsoft. I will accept that if you use the Dockerised version, Docker or GitHub will get that data instead. None of this is "telemetry", e.g. detailed usage information. It's quid-pro-quo; you want to know if there are updates, you ask the update server, it records that your IP looked for updates.

Jellyfin Server collects data and stores it locally on your device in logs and its database (e.g. viewing data, login sessions). This data doesn't leave your device.

What Jellyfin Server _will_ share a lot of, is data about your collection, to its metadata providers. You chose those when setting up your libraries. In return, they hand back nice metadata and pictures. So TheTVDB.com, OMDB, MusicBrainz and others you've configured, get queried about "Film name (year)" or "TV series S01E01" or "Band name Album name" and thus learn everything that's in your collection... but not whether or when you've watched it.