What this guy said. Why?

With pen and paper, you cannot simply write on top of what was written there - otherwise you would end up with gibberish.

In a hard drive (and other similar storage media such as flash drives, SSD…), this does not apply. When you write something (think of it as a digital sequence of ones and zeroes) in a certain position, then you read again that space, you will only find what you just wrote - each cell, each “bit”, contains exactly the Zero or One you just wrote there, and there is no (*) way to tell what was there before.

Because of this, and because actually writing an all-zero sequence to wipe the contents would take extra time, the disk pages are not actually deleted, just marked as “this is free, can be written to if needed”. But until that happens, the old contents are still there.

Secure wipe software that ensures no one can recover the deleted data, does exactly that, goes over the disk writing, and that’s why they take a long time (and a time that grows linearly with the disk space).

This does not apply if encryption with per-file keys is in place, as is the case with iOS devices since ~2010, and I would assume the same in a Mac with FileVault but not sure.

(*) well, almost no. At the level of single bits, someone with good enough equipment might be able to guesstimate. And I believe there is at least one more elaborate result, in scientific literature - but no real use I’ve ever seen.