Hard disk and cd disc storage are not the same things. Hard disks are called disks because they use magnetic technology, and cd discs use a laser.
Because it is stored in ultraviolet light, it is considered a disc. Information is said to be stored digitally, but on this platter, it is stored as an analog signal. In other words, digital signals are analog signals. Once you want to read the data on your hard disk, you need to convert the analog signal into a digital signal.
Did you mention that there are magnetic fluids? Assume that there are a lot of small magnetic blocks on these platters. When you say magnetic blocks, there are north and south magnets. There are a lot of these little magnets. For example, if you take ss to get 1, then SN is going to take 0. This is called the magnetic bit. There are hundreds of millions of small magnetic blocks on this platter. As I said before, you have to convert the digital signal back. In this way, the data is stored and read. Hard disks have been around for many years, and to this day their structure has not changed much. The basic structure is the same.