To add to your point, Microsoft had originally envisioned that vendors would write their own HAL.DLL’s to make Windows compatible with their hardware, but it turned out nobody really had the expertise needed to do that, and Windows started to depend more and more on implementation details of the most common one, and at this point the idea is dead. Hardware vendors target one single windows HAL for x86_64, and porting windows to another platform requires rewriting/rebuilding all sorts of stuff, not just the HAL. It was a cool idea at the time that didn’t really pan out.
There was a certain level of custom HALs being written, but those depended heavily on the "shared development" model that Microsoft did with vendors, and a lot of hardware was shipping bog-standard PCs anyway.
The rare custom HALs were mainly for things like x86 NUMA systems, or from ports to alternate architectures.
