Freeing up space · What's Using Space (Storage Inspector)
Are the sizes in What's Using Space measured or estimated?
Short answer: Measured on your Mac, not estimated. Where something can't be measured you're told so, and the footer says exactly what the figures do and don't cover.
Measured. Every figure is read from your Mac, and where something genuinely can't be measured, Storage Studio says so instead of filling the gap with a guess.
The footer is explicit about coverage: if parts of your disk are off-limits to every app, or measuring stopped early to stay out of your way, it tells you what the figures do and don't include. Measure again walks your disk afresh, and results appear as they arrive rather than all at the end.