Taking a quick look at the source code (of which I am not familiar so I could be barking up the wrong tree), it seems that Jellyfin gives you a list of candidates in the following order: similar to recently played, similar to liked, directed by recently played, actor from recently played, has liked director, has liked actor. Similarity is calculated by rating and production year, as well as shared number of connected people, genres, studios, and tags. For music, it also adds specific criteria for other albums the artist has worked on.
Taking a quick look at the source code (of which I am not familiar so I could be barking up the wrong tree), it seems that Jellyfin gives you a list of candidates in the following order: similar to recently played, similar to liked, directed by recently played, actor from recently played, has liked director, has liked actor. Similarity is calculated by rating and production year, as well as shared number of connected people, genres, studios, and tags. For music, it also adds specific criteria for other albums the artist has worked on.