Unfortunately, there are no granular and accessible backups in iOS or iPadOS. The backup to iCloud is meant only for setting up or restoring a complete mobile device.
At this time, while sync is not intended as a backup, it may be used as a current backup of a database. Since the data is only useful to DEVONthink and DEVONthink To Go, it is not a true, product-independent backup, though.
Alternatively, you can export databases a files-in-folders to the Files app and back them up from there.