If you have more than one Photos library the macOS will use one of them for the “System Photo Library”. Recently, I was combining a couple of Photos libraries and needed to change the “System Photo Library” to a the combined library.
Change The System Photo Library
What is the “System Photo Library?” It is what the macOS considers to be THE photo library for your computer. So, if you take some photos on your iPhone or you import photos from a SD Card they automatically go into the “System Photo Library.” If you have only ever had one Photos App Library you do not need to worry about this. However, if you have multiple libraries and you need to change which photo library is the “System Photo Library,” then do the following. If I open the Photos App holding down the Option Key ⌥, I get a window to choose which photo library I wish to open:
In this case the macOS considers the top listing as the “System Photo Library.” I want it to be the library titled “Old Photos Library” instead. So, with this window open I select “Old Photos Library” and then click “Choose Library” to open the new library.
Once I have this library open all I have to do is go under the Photos Menu and select Preferences:
Just click the box titled “Use as System Photo Library” and this current open library will be your default library going forward.
Changing your System Photo Library to a different set of photos is pretty easy. It is not something you would need to do very often, but it is doable if you need to do so in the future.