How does Backupper detect file changes in Image backup
I am trying to figure out a way to backup a Veracrypt data. When I backup a Veracrypt partition, I notice that backupper is unable to integrate with Veracrypt and do a sector by sector partition. This is not desirable size-wise, a 5Tb partition that is mostly empty will result in a backup that is 5TB in size.
Instead of using a encrypted partition, I leave the partition as exfat and then create a giant veracrypt file container. What we end up with is a 5 Tb veracrypt file. I then proceed to image backup the volume. What I have notice is that while the backup is really slow (it has to process 5 TB of data, so it's going to take like 24 hours), the resulting backup only contains actual used data. A 5 Tb container with 200 Mb of files will result in a 200 mb backup. If you add another file, and run increment or differential backup, only what was changed will be backup (but still takes like 24 hours even if the file is only 1K). I have tried restoring and it appears to work so far.
My question is how does this work? How does backupper notice the change in the veracrypt file? What could cause the backup to break?