Upload videos to imovie

If the Photos app library is designated as your system photo library in the Photos app general preferences, then you should be able to access the videos in iMovie from the Photos item in the iMovie media sidebar. Here's the Help menu item on that:

https://help.apple.com/imovie/mac/10.1/#/mov865570171

This would not work for videos that are stored in the Cloud, and might not recognize some videos if you are using iCloud Photos and have the optimized storage option selected.

Also, you can open iMovie and the Photos library at the same time and drag directly from the Photos library into iMovie, either into a project timeline or into an Event.

EDIT: Some legacy media in old and now unsupported formats may not import into iMovie if you are using Catalina or Big Sur.

-- Rich

No, videos do not have to be dragged to the desktop before importing into iMovie. What steps are you taking to download the videos and what happens when you try? Where are the videos located?

-- Rich