Apple has been using the APFS file system on Macs for a couple of years. It has finally reached its full potential. And, in macOS Big Sur, the APFS file format actually works properly with the Time Machine backup software. Time Machine in macOS Big Sur backing up an APFS volume is way more efficient than the old backups using the HFS+ file system. This article is somewhat technical, but it does explain why Time Machine in Big Sur works so much better than previous versions of macOS.