Where are the items I imported into Photos on my Mac?
When you first use Photos, you create a new library. New photos and videos that you add to Photos are stored in this library. The library is located in the Pictures folder on your Mac.
The library automatically becomes your System Photo Library, which Photos accesses each time you open the app. Only your System Photo Library can send or receive photos and videos transferred by iCloud Photos.
You can also create additional photo libraries on your Mac and on other storage devices. You can use as many different libraries as you like. You should only use the Photos app to access the photos and videos in a library.
WARNING: To avoid accidentally deleting or corrupting a photo library, do not manually access or alter the contents of a library in the Finder. If you want to copy, move, or transfer files, first export them from the library.