There doesn’t appear to be an existing tool that automatically migrates an Apple Photos library into darktable.

The main limitation is Apple Photos itself: its AppleScript support is quite limited, so you likely won’t be able to extract all EXIF/IPTC metadata or all Apple-specific fields that way. In practice, the safest and simplest route is usually to work from the original image files in folders rather than trying to fully translate the Photos library structure.

If you want to build your own importer, Apple Photos stores library data in Pictures/Photos Library.photoslibrary/database, with much of it in Photos.sqlite. That database can be inspected with an SQLite browser when Photos is closed, ideally using a copied database file to avoid accidental changes. However, doing this will require understanding SQLite plus some internal details of both Photos and darktable, and the database schema may change between macOS versions.

So: no ready-made migration path is known here, and full metadata/edit migration may be incomplete. Exporting originals from Apple Photos and then importing those into darktable is likely the most practical approach.