I’m using macOS High Sierra. Recently I started to shoot movies and use FCPX to edit etcetera. My question is when going to About this Mac > Storage > Manage > Reduce Clutter> Review Files > Large Files… Much to my frustration I find oodles of QuickTime movie files. I’d like to delete them and regain the almost 300 GB of storage that’s being used up. But I’m worried they would no longer be stored in my Photos application (where I imported or dragged and dropped everything I’ve filmed). Why do they end up saved in that documents file and is it safe to delete them. A related question, when I delete downloads, if I’ve saved them in photos or in pages, will those be gone from those places or will they remain?
This largely depends on how you have photos imported to the Photos app. Photos > Preferences has an Importing checkbox. If this checkbox was ever disabled when you imported photos, you may not have all photos in the library itself and may exist in other places on your computer.
If you are not using iCloud Photo Library, consider selecting the image or movie in Photos, and using File > Show Referenced File in Finder. This will allow you to see exactly where the file lives.
You could export all your movie files from Photos and move them to a different hard drive or cloud storage solution, or delete the movies in Photos and replace them all with the Importing checkbox selected. You should export originals via File > Export > Export Unmodified Original for x Videos.
I also recommend making sure you have good backups via Time Machine or another service. If you make a mistake when deleting files, you can recover files.