I know this is software focused, but as much of the work done in DOS 2.0 was to support the introduction of hard disks, it would be worth mentioning what the hardware was in the PC/XT:
https://en.wikipedia.org/wiki/ST-506/ST-412
https://www.google.com/search?client=firefox-b-d&q=ST-212+ha...
I love that for the next few years they were commonly called Winchester drives due to IBM's original naming in the early 70's.
As I recall 8 inch class HDD (5, 10, 20MB?) were called Winchester in the pre-PC/XT era, named for the earlier IBM product, and then the terminology carried on into the microcomputer/desktop world when the 5.25" full height HDD was introduced for the XT.
My dad still called it "Winchester" well into the 80s. I remember when we got our first "Winchester", a 20MB hard drive.
My first hard drive was 6Mb. I had to design/build my own interface board for it, and write the driver for RT-11.