To trim some video from the beginning or end of a movie file, you do not need to use a video editing app. You can do it right in QuickTime Player. Trimming is loss-free and can be done with precision. You can also cut sections out of the center of the video and export a new copy. You can also trim in the Photos app, although the full-length original video is always available if you want to go back.
Check out easy trimming of YouTube QuickTime Player video files for closed captions and more options.