Where are my photos?
By default, the photos and videos you import into Photos are stored in the Photos library on your Mac.
However, if you import items from a storage device or another folder on your Mac, you can have Photos leave the items in their original locations so they don’t take up extra space on your Mac. As long as the device or folder is accessible to Photos, you can view and work with these items in the Photos window just as you normally would. (Files that are stored in another location outside your Photos library are known as referenced files.)
Although storing files outside your Photos library is a great way to save space on your Mac, be aware that:
If you use iCloud Photo Library, photos and videos outside your Photos library aren’t stored in your iCloud Photo Library. So these files won’t be accessible to any other Mac or device that uses iCloud Photo Library.
If you disconnect the device where the files are located, or move or rename the files in the Finder, Photos won’t be able to locate them, so you can’t work with them in the Photos app.
Note: If you use iCloud Photo Library, another good way to save space is to turn on the Optimize Mac storage option in iCloud preferences. iCloud Photo Library then places smaller photo files on your Mac as needed. All the full-size photos and video will always be accessible to your Mac from iCloud, but your Photos library will take up less space on your Mac.