Excuse my ignorance but how does OS development even work? Being a mostly web developer, it just seems to arcane to change an OS - like, pre-application layer.
Not unlike web development, you're working against a contract (in this case, CPU instructions/features, storage formats, bus formats) defined by some megacorporate legacy. You also have support from pieces of embedded firmware running inside storage devices or other peripherals.
The book at http://ostep.org gets into some details.
Another useful illuminator: https://gwern.net/computers
As much as possible is generally done using simulators before testing on any real hardware.