You'd presumably have some "block device" abstraction between your filesystem and your device driver. Don't want to re-implement a FS for each type of hardware. On a Linux system, you can read, eg, /dev/sda1 from userspace, which is what it looks like this filesystem probably does.

As for how you actually request data from the hard drive: There's older ATA interfaces, and BIOS routines from them, which I suspect is what most hobbyist OSes would use.

A more modern interface is AHCI. The OSDev wiki has an overview, where you can see how the registers work: https://wiki.osdev.org/AHCI