With images and videos that you want to index, note, modify, and manage, Photos for macOS has a powerful option that lets you store that media in its original locations, rather than copied into the Photos Library. In Photos > Preferences, in the General tab, simply uncheck Copy Items to the Photos Library.
Any media that’s referenced instead of copied will appear with a special confusing little icon that looks like an arrow pointing outside of a rectangle.
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A tiny icon on referenced images indicates they’re not copied into the Photos library.
