How macOS actually works
APFS replaced HFS+ in 2017. Four of its features explain most of the confusing storage behaviour people run into.
A physical disk holds a container, and a container holds volumes that share its free space. This is why Disk Utility shows several volumes each claiming the same free space — they are not lying, they genuinely share it.
Your Mac normally has at least Macintosh HD (read-only system) and Macintosh HD - Data (everything of yours).
Copying a file within the same volume does not copy the data. APFS records that both names point at the same blocks, and only writes new data when one of them changes.
Practical effect: duplicating a 10 GB video appears to take no space and no time. Both files are real, and space is consumed only as they diverge. This is why "size on disk" figures often refuse to add up.
A snapshot freezes the state of a volume. Time Machine takes them hourly on the local disk before copying to your backup drive. They hold space until purged, which is a common cause of "where has my free space gone".
tmutil listlocalsnapshots /The default APFS format is case-insensitive but case-preserving: Report.pdf and report.pdf cannot coexist, but the capitalisation you typed is remembered.
A case-sensitive variant exists and is occasionally chosen for development. Some apps misbehave on it, so it is not the default for good reason.
For search this matters: on a normal Mac, matching filenames case-insensitively is correct behaviour, not sloppiness.
APFS has no equivalent of Windows' NTFS Master File Table — no single structure listing every file that a program can read in one pass. Any tool building a complete index has to walk the directory tree itself. That is why a full first scan takes tens of seconds where the Windows equivalent took two.
Not by default. macOS uses a case-insensitive, case-preserving variant; a case-sensitive format exists but is uncommon.
They share one APFS container. Free space is pooled rather than partitioned.
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