iCloud Drive keeps re-downloading a 40GB file
There's nothing that appears to be 'fixed', based on the description in your question.
The purpose of iCloud is give you access to your data from any device logged-in with the same Apple ID. Using iCloud, you can access photos, files, contacts, Safari tabs, etc, just like they were part of your file system.  For example, if you create a doc using Pages on your Mac you can access and edit that same file from your iPad using the Pages app.
It looks like iCloud is simply syncing data between your devices at a regular interval.  This is normal, expected behavior.  You can control what's synced by going into your Apple ID via Settings on any logged-in device.
If you only want a backup solution, you can look into other services such as Dropbox and OneDrive.