Mac storage basics · When the disk is full
Why do I have less free space right after a macOS update?
Short answer: A pre-update snapshot, leftover update files and rebuilt caches and indexes. Most settles within a day or two; delete any leftover Install macOS app.
After an update macOS may keep a snapshot taken before the update, leftover update files, and rebuilt caches and indexes (Spotlight and Photos often re-index). Most of this is temporary and settles within a day or two.
If space stays low, check for an Install macOS app left in Applications and release old local snapshots. See local snapshots.