When you save data into your computer the data is written to the hard drive through the magnet and that needle like thing that sweeps back and forth on the hard drive platters.
The data is then arranged in concentric, circular paths called tracks on the hard drive even though you cannot see those bits of information.
Each of those tracks on the hard drive are broken up into smaller areas called sectors where the information is stored in a jumbled data form that would look very confusing if you actually saw it.
The hard drives store the data in code known as binary code; using 0s and 1s and the hard disk is magnetic allowing the data to be spread out on the magnetic layer of the hard disk.
The hard disk drive has heads that "float" above the surface thanks to the layer of air produced by the ultra-fast rotation of the hard disk.
It's amazing how all that information can be stored on a metal disk even though you cannot actually see the data.
Such as all those photos and videos are on the hard disk of my computer that I can pull up on my computer screen and view them within seconds.
But if you took the hard drive apart you cannot see the pictures even though they are there.