Best way to recover a photo library when it is incompatible with current os

I’d like to delve deeper into recovering a Photos library. The originals are very easy to get back. How/where is stuff like added locations and facial recognition/people data stored, and now pets. I’ve done a ton of work that I’d really like back. Thus I’d like to know what all the package contents are. Which files/folders/contents, besides originals/masters can be copied between libraries? Which ones contain data related to modifications and augmentation of originals? Are system changes resulting in compatibility issues interlaced with data (I really hope not)? The Sonoma update messed up my iMac, which became non-functional and the end result was a clean install, reverting back to Ventura via Catalina. It is stable again and I’ve restored everything but Photos. If Apple isn’t going to provide compatibility, I want to document a process for myself to know I can get back all my stuff just in case this happens again. Too much work in Photos to lose it. So what is that process for photos? Step one, get image files you loaded. Next?