What we save in your photos, and who reads it

No. Google Photos groups faces on its own and ignores names saved in a file. The names stay in your file and in Timeline Scan, and Immich receives them over our connection.

No. Every detail is written into the photo file itself in standard fields. An app that ignores one today may read it later, and any app you switch to gets the whole file.

Yes, all of it: the date, the order, the note, the names, the place, the rotation. Your corrections are what gets written into the file.