When I trim Slow motion video using iMovie video disappear from phone and iCloud

If you want to import the edited version of a movie to a Mac or PC that doesn’t automatically import the edited version, here's what to do:

AirDrop or email the movie to yourself.

Or, if you've enabled iCloud Photos, you can download the movie from the Photo Library web app at iCloud.com.

These actions give you a copy of the edited version on your computer, but you won't be able to revert to the original, unedited version.

https://support.apple.com/en-us/HT204064#photos