If you have multiple Photos libraries, at any given time there will be only one that is designated as the System Photo Library. This library is the one that is made available by macOS for other applications to access directly. This library will appear in the photo browser in apps like Mail, iMovie, and Pages, as well as other services like your screen saver settings. Applications that offer to import into Photos will import into the system library. The system library is also the only library that can be synced with iCloud.
To specify which library you want to be the system photo library, open the library in Photos, open the settings window, and click the “Use as System Photo Library” button. (if the button is disabled, that means the library is already the system photo library) Simply opening a library in Photos will not switch the system photo library, so you can feel free to open and work with other libraries without changing which library is set as the system photo library.
For more information on the system photo library, visit this support article on Apple’s website.