Face ID does not transfer from one Library to another-- you have to wait for a device to scan all your pictures and try to identify faces. This can take quite a while. And, on a Mac, you can add a name to faces that Photos has not recognized as faces. You do this in the Info Window
Once you put the circle on the face, you can name the person
maybe "Prince of Wales" or "Charles Windsor," and you can even change the name later to "King Charles III."
For family names, I also include a keyword, like "KingCharlie" or something. Keywords transfer with pictures, but face IDs do not. Face IDs can be wiped out in an OS update, but keywords persist. Keywords don't appear in iOS or iPadOS, but they are searchable. So, on a Mac, you can search by keyword or person, or both, to find people. You can also use the People view to make sure that they all have proper keywords, and you can collect keywords to make sure you've face ID'd them all. I also put name is Captions, because those do show up in iOS and iPadOS.
I'm not absolutely sure, but I think the only place I find Face ID especially valuable is in Family Reunion pictures with 12 people in it, 7 of whom I won't remember next time I look at the picture. Circling faces and writing their names is very useful for those group pictures, but for most, a simple list in the Caption seems sufficient. Captions can be very long, by the way, like paragraphs, but searches only see the first few words. In old family pictures I use the Caption to list the names, birth and death dates, relationship to me or my wife, and maybe even little stories.
