How does Delta Restore work?

[Haloes]

Although I've been using backup/restore software for decades (Macrium and Hasleo) I'm puzzled about how Delta Restore works.
I only use Full Backup and therefore Full Restore options since my current C: is only a 512GB SSD.
Backup takes < 5 minutes writing to an external SSD.

So Delta Restore only restores the changes in the file system.
After a backup, and before the restore, a file system will have had many changes including sector positional changes.
How does it know which sectors haven't changed.

Can anyone enlighten this "relative" newbie?
Thanks.

[CDC9762]

From their site:
Delta Restore in Hasleo Backup Suite is a smarter and more efficient data restoration technology. It works by comparing the data in the historical backup with what currently exists in the original location, retaining all unchanged data and only restoring the changed parts.
Obviously, a comparison basis makes sense and also this is why changes in the size of partitions from the image to the current partition will not permit a delta restore.