.MOV videos will not open on QuickTime or iMovie

In the Photos app library that that you have designated as the system photo library, you can go to Photos/Preferences/iCloud and enable iCloud Photos. That should back up your system Photos library. However, it isn't really a total backup but rather a syncing feature.

I suggest that you post your question in the Photos for Mac forum, as the contributors there are very expert in the use of the Photos app. I do not use iCloud Photos so not completely familiar with the subtleties of it. Here's the link:

https://discussions.apple.com/community/mac_os/photos_osx

-- Rich

Try converting your .mov clips to H.264, Mp4/AAC with the free download, Handbrake. Handbrake will do it by default. Do that even if the codec already is indicated to be Mp4/AAC. Re-rendering often cures the issue.

You can get Handbrake here:

https://handbrake.fr/

A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.

-- Rich

Here's a screen shot of the Photos app preferences box re iCloud:

-- Rich

Great. Have fun. 😊

-- Rich