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. There are no versions in the backup. The synchronization updates and replaces the sync data, so this method would only be useful in very specific situations. Also, since the data is only useful to DEVONthink and DEVONthink To Go, it is not a true, product-independent backup.
Until we add the planned export function you can also sync to DEVONthink on a Mac and export to files-in-folders there.