To backup your photos, you need to use the Google Photos App. To set up automatic photos and video backup, launch the Google Photos App and follow these instructions:
If you see a message asking to allow the app to access your photos:
You can read more about the Photos Backup and Sync here:
You will not be able to save iMovies that are in progress. You must export the iMovie as a finished project to the Camera Roll for it to be backed up to Google Photos.
After Google Photos is setup, while connected to a WiFi network, launch Google Photos and it will automatically backup any new photos. This is a good way to always make sure your photos are automatically backed up. Remember to launch Google Photos on a regular basis.
The process of setting up Google Photos is illustrated below: