Clarification of photos metadata and how “everything” is stored in the photos library

“Everything” in photos to me includes the originals, modifications to the images, augmentation with people identification and location specifics, titles, captions, comments - anything you can add in the “get Info” screen.

Important to note that you can export originals and you can export edited versions, but they will be two separate files.

From what was said about export, people wont be part of that. Is that correct?

Correct. But there is an easy workaround: use the names as keywords as well.

That said, can the images and the side car files BOTH be extracted from the library if you didn’t export?

Not without exporting. The metadata and editing instructions are stored within the database.

Since “people” is proprietary, can I extract it from the library to use in a new library on an older os?

No. Proprietary isn't the issue. Exif and IPTC are standardised metadata there is no agree standard for sharing this information.

If not, could it be done by an apple engineer?

Perhaps.

Is it stored in the same file or are there additional files?

In the database. The reason being that it's a lot faster to search one file with 80,000 entries than 80,00o individual files.

No earlier version of Photos can open a later database. This was equally true of iPhoto and Aperture. All of these apps - and by that I mean parametric editors, like Lightroom, On1, CaptureOne et al - work the same way. Obviously the ones you pay for are more powerful than a freebie like Photos, but it's essentially the same. The ultimate protection is a back up made before you update. One that is updated constantly. That's the fix.

And, keep in mind, the Photos Library is a bundled package, that only Photos can read or some third-party apps like Power Photos. Even if you open the package in Finder and look at the parts of the library, it will be very hard to find even certain photos in the library package, as Photos does no longer store the original image files with the original filenames, but is renaming them. To be able to access all photos inside a Photos Library including the metadata and the original filenames, we need a Mac and a version of Photos that can open the library - the same version that created the library or a newer version.

The only way to downgrade a Photos Library to an older version of Photos is the detour via iCloud Photos.

Make a full backup of your Mac as it is now, so you can easily revert to this version - then upgrade again to Sonoma, open the Photos Library in Photos and enable iCloud Photos. Once your complete Photos Library is in iCloud Photos, you could revert to Ventura from your backup.

Now create a new, empty Photos Library, make it the System Photos Library, enable iCloud Photos, wait for all photos to come back from iCloud. And you will have a downgraded version of your Photos Library. See: Downgrading a Photos Library to a Previous Version of Photos with the Help of iCloud Photos