How do you organize spin-off movies and TV shows?

[TheDreadPirate]

Another option that is, IMO, simpler is to use playlists.

Jellyfin is pretty "dumb" when it comes to organization and is 100% reliant on how TMDB/TVDB structures things. And that is each show and movie are separate entities. The way things are fundamentally structured does not allow for the kind of organization you're describing. At least not in an automated way. There would need to be a metadata source that would group a spin-off under the same entity as the show it was spun-off from. And has an api and structure that isn't so foreign as to not be compatible with the "Seasons" and "Specials" structure of TV shows in Jellyfin.

Jellyfin would need quite a significant overhaul to be able to handle nesting a spin-off under the originating show with the available metadata sources.

Jellyfin just shows you what you have. If a franchise of shows and movies needs or should to be watched in a particular order, playlists are the way to go since they allow for arbitrary ordering of show episodes and movies.