How can I see where Photos.app thinks the System Photo Library is? I have about 8 photo libraries, all of them on a [connected and mounted] external disk. One of them is the System Photo Library, but which one? I have forgotten.
Since I upgraded to Big Sur, Photos.app is saying it cannot find the System Photo Library. Apparently, Photos.app is looking for it in some place where it is not.
I would like to know where it is looking, and what is the name of the library it is looking for, so that I can point it to its correct location.
Necessarily, Photos.app has this information stored somewhere. Where?
In Photos if you go to preferences (⌘ + ,) under General there are some library location settings:

In order to find out where System Photo Library is and to change it just hold ⌥ while opening Photos. You will be presented window with needed controls:

Refer to Apple documentation for needed steps: Designate a System Photo Library in Photos