Some APFS links:
"APFS is a more complex and dynamic file system than HFS+, in that there often isn’t a simple answer to how much free space there is on any given disk or volume."
https://eclecticlight.co/2020/04/09/where-did-all-that-free-space-go-on-my-apfs-disk/
"Snapshots are one of the major features of APFS"
https://eclecticlight.co/2020/07/13/when-the-finders-arithmetic-fails-with-an-error-of-139-tb/
"One of the much-touted features of APFS is its ability to ‘clone’ files which are copied or duplicated. It’s easy to demonstrate this with a suitably large file of a few GB or more. Select the large file and drag-copy it to a different volume, noting how long it takes to copy across. Then select the same large file and duplicate it in the Finder (⌘D), which is instantaneous. The duplicate pretends that it’s a completely new file, but we all know that duplication simply can’t occur in such a short space of time."
https://eclecticlight.co/2021/04/02/how-can-you-tell-whether-a-file-has-been-cloned-in-apfs/
"If there’s one thing we’re learning about APFS, it’s that file sizes are flexible."
https://eclecticlight.co/2021/03/29/sparse-files-are-common-in-apfs/