iMOVIE

There are three ways to move a video or photo from your Photos app library to iMovie:

1. Drag the video or photo directly from the Photos app library to the timeline of an iMovie project. (Not sure if that option is still available for operating systems after Mojave, but worth a try.)
2. From the Photos app library, do a File/Export of the video to your desktop, and from there drag it into an iMovie timeline.
3. Click on the Photos item in the iMovie media sidebar of a project and you can display your Photos app videos and photos in the media browser. From there you can drag them into an iMovie project. (This is the recommended way.) For this to work, your Photos app library must be designated as the system photo library in the Photos app General Preferences. It is designated as the system photo library when the option "use as system photo library" is grayed out by clicking on it.

-- Rich