It was a zip file that included a Read_Me_First.pdf and 8 JSON files.
The 6 page long PDF is just some information about your GDPR and California privacy law rights, and contact information to request more information such as technical log information, extended streaming history, or other special data they collect to provide and troubleshoot the service. It's 6 pages because they have that information in 21 languages.
The JSON files are named Follow, Inferences, Payments, Playlist1, SearchQueries, StreamingHistory0, Userdata, and YourLibrary.
Playlist1 appears to contain all of my playlists, and would be sufficient for recreating them on another service.
For the curious, I'll describe the rest in a comment.