No the example under Edit 2 is Btrfs. On the details I'd expect APFS to differ. But the gist is that how changes manifest on disk do also depend on the applicat

No the example under Edit 2 is Btrfs. On the details I'd expect APFS to differ. But the gist is that how changes manifest on disk do also depend on the application being used. On any file system, an edit in vi means a whole new file is written out, so it's effectively COW at an application level. Whereas an edit with an application that does byte level overwrites, would get translated into COW on a COW file system like Btrfs, APFS or ZFS. And I'd expect this COW operation to operate at the file system's minimum block level size.