Mac storage basics · Caches, logs and temporary files
Where are temporary files on a Mac, and do they clear themselves?
Short answer: In hidden per-user areas under /private/var/folders and in /tmp. The temporary area clears on restart and macOS empties the cache area when space is tight.
macOS gives each user a private temporary area and a cache area inside a hidden system folder (under /private/var/folders), plus the shared /tmp. Apps write scratch files there expecting them to be thrown away.
The temporary area is cleared when you restart, and macOS empties the cache area itself when space gets tight. Problems arise when an app leaves large leftovers behind and the Mac rarely restarts.
Don't delete inside these folders by hand while apps are running — some files are in use. Restarting is the safe fix. Storage Studio measures these areas, attributes leftovers to the app that made them, and only offers to clear folders that are not open, not recently touched and recoverable. See freeing up space.