Help with iMovie on my phone?
Store your videos on Google Photo. The app for your phone is free and storage is free, up to a large limit. However, you'll have to keep the clips that you are actually using for a movie on your phone until you complete and save that movie. Then you can delete them, assuming you have them backed up on Google Photos first.
No. iMovie gets the media from the Camera Roll. You could place them in iCloud storage until you are ready to do whatever or you could do whatever with them to clear them out faster.
Store the video clips outside of the phone - like iCloud. Lots of video storage will require a subscription fee.