Check iMovie > Preferences > Theater and see if
Automatically Upload to iCloud is checked. I have used iMovie on the Mac a few years but recently I have only used it on my iPhone or iPad. obviously there have been a LOT of changes on MacOS so it took me a while to figure this out. When you have a "Project" complete and ready to go, you click on the "Share Icon" (a box with an upward Ponting arrow at the far right end of the iMovie tool bar) and you are offered the options of
- Theater
- Email
- iTunes
- YouTube
- FaceBook
- Vimeo
- image
- File
Choosing one of the options renders the "movie" into an appropriate format (which will take a good while) and sends it to the chosen location. iMovie Theater is a Library found in your home folder and if the preferences are set properly it will also upload to your iCloud Drive. Other than that, as far as I can tell iMovie media may be drawn from any source on either local storage, servers, and iCloud, but the Projects remain on the Mac.
iMovie on iOS, on the other hand puts just abut everything on iCloud. Why iMovie is different, you will probably have to ask the developers.
