If you open Photos > Preferences with Photos connected to the current system library then the option to use the current library as the system photo library will be greyed out. See Designate a System Photo Library in Photos - Apple Support for details on how to access a different library and set it as the system default. Your designated system photo library should appear in the sidebar in iMovie. See Add Photos images to an iMovie project - Apple Support for more. Dragging in images from Finder may be easier than locating them through this media library list.
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