Answers...

to commonly asked questions.

Switching between Photos libraries

I have ‘3 different large folders’ on my photos. And right now I’ve only got 4-5 years….(because of this OLD mac!) I think there are 3 keys you can hold down together to open one of the other large folders.
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If I understand you correctly:

folders=libraries

Hold down the OPTION key before you start Photos. It will give you the opportunity to choose which library you want to open. Photos remembers the last library your opened. So you may have to hold down OPTION every time you open Photos if you plan on going back and forth between libraries often.
This image is a theme.plist hack