There are a few applications that don't automatically backup to the iCloud unless you turn it on or save the items you want someplace else. See below for instructions on how to do this with each app.

Notability

Select the settings gear

Select iCloud, then turn on the toggle for iCloud

Just to be safe, you can also turn on an extra backup to Google Drive by selecting Auto-backup, then Google Drive (if this is your first time doing this, it may ask you to sign into Google Drive)

Book Creator

Slide to the book you want to save and select export (square with up arrow)

Then select Export as ePub

Select Add to iCloud Drive, then select iCloud Drive again

To be safe, you can also add it to Google Drive. Repeat the steps above and instead of selecting iCloud Drive, scroll over to select Copy to Google Drive as shown below

iMovie

Select the project you want to save, then select export (square with up arrow)

Select "Add to iCloud Drive"

Next go back to select the export button again, now select "Copy to Google Drive"