How could updatedb run quickly by avoiding doing full disk scans (maybe not mtime)

How is it possible for updatedb to run very fast by avoiding doing full disk scans? Since mtime of parent folder is not affected by files in subfolders. In Why does the updatedb program run so fast? > @hugomg I don't think it actually does. It should only change the > mtime of its immediate parent. – @Kusalananda I searched but cannot find the answer.