Where are Photos projects stored on the drive?

Solution 1:

Here is the official answer from Apple (specifically, Jack, a senior advisor with Apple Developer Program Support):

After reviewing both public and Apple Developer support documents, the most transparent answer I can provide here is that if these files were directly accessible and transferrable in the way that you are asking, that information would be documented. In reviewing our documentation, I found that all files and subsfolders associated with the Apple Photos app are packaged in the user>Pictures>Photos Library.photoslibrary file. As the structure and contents of this package are not defined in any Apple Support documentation, Apple Support will likely not be able to provide the specific answer you are looking for.