On devices with full-disk encryption (such as iPhones made in the past few years), a "factory reset" is equivalent to scrambling the entire disk. This is becaus

On devices with full-disk encryption (such as iPhones made in the past few years), a "factory reset" is equivalent to scrambling the entire disk. This is because it securely erases the keys that were used for all the data on the disk, and without the keys, all that data is effectively random garbage.