You can delete your Projects in iMovie but can’t delete the videos in the frame by frame list because there not in iMovie yet. You need to delete them in the photo app then they will not appear in the list
Videos in the Projects list can be deleted by opening the project and selecting the trash can to delete. Videos in the Video list can’t be deleted. Those aren’t completely associated with what’s in the camera roll either, for example when the video is opened into iMovie from another app. The only way I’ve found to delete those is to uninstall iMovie and reinstall it. You could delete them in older versions of iMovie, but that option disappeared a few years ago.