Photos in your System Photo Library are available in apps like iMovie, Pages and Keynote. You can also sync them to iOS devices and Apple TV via iTunes. And if you want to use your own images as your desktop picture or screen saver, the images need to be in your System Photo Library before you can select them in System Preferences.
If you have only one photo library, then it's the System Photo Library. Otherwise, the first photo library that you create or open in Photos will become the System Photo Library. If you have more than one library, you might need to designate a System Photo Library, so other apps can access the photos and videos you want them to use.
You can access the hidden Library folder without using Terminal, which has the side effect of revealing every hidden file on your Mac. This method will only make the Library folder visible, and only for as long as you keep the Finder window for the Library folder open.
2020-4-4 How to create entirely separate photo libraries in Photos. By Rob LeFebvre. 2:00 pm, April 16, 2015. Screenshot: Rob LeFebvre/Cult of Mac. To create a new photo library, click on the Create. All your photos on all your devices. ICloud Photos gives you access to your entire Mac photo and video library from all your devices. If you shoot a snapshot, slo-mo, or selfie on your iPhone, it’s automatically added to iCloud Photos — so it appears on your Mac, iOS and iPadOS devices, Apple TV.
Follow these steps to designate a System Photo Library:
If you open a second or different library in the Photos app, and you haven't designated it as the System Photo Library, other applications will use photos from the original System Photo Library. Hold down the Option key when you open Photos to see which library is set as the System Photo Library.
You can use iCloud Photos, Shared Albums and My Photo Stream only with the System Photo Library. If you choose a different library in Photos without designating it as the System Photo Library, the iCloud tab in Photos preferences is disabled:
If you designate a new library as the System Photo Library and then turn on iCloud Photos, the photos and videos in the new library will merge with those already in your iCloud Photos. If you want to keep the contents of your photo libraries separate, don’t turn on iCloud Photos for more than one library in Photos.