1) ARC boot firmware. NT was developed on non-x86 systems like i860 then MIPS, and ARC is the native boot firmware used (on x86 NTLDR emulated it prior to Vista). Similar to a PC BIOS / UEFI, and a PCI PowerMac would use OpenFirmware. As well as providing an ARC compatible environment that loads over OpenFirmware, this project seems to does some fun so the boot firmware can pretend there's a storage device so that driver "floppies" can be loaded during the initial stages of setup. (ie an F6 disc)
2) A HAL. The main NTOSKRNL is hardware agnostic, so the idea is that there's one binary for each CPU architecture. But the kernel needs to interface with actual timers, busses etc., so the interface code is in HAL.DLL and the appropriate one is copied by setup. (For example see https://www.geoffchappell.com/studies/windows/km/hal/history... for a list of x86-32 ones in older versions of windows, with various HALs for NEC PC-98, IBM MCA, various early multiprocessing systems, nowadays there's just one AMD64 one that's mostly in the kernel itself). So the main kernel in unaltered, and halgoss handles the Mac specific stuff.
3) Device drivers. Once NT is up and running it does need actual drivers.
The specs for 1) are known, so can presumably be emulated, and I guess there's a DDK for 3) (or it can be deduced from another DDK), I guess 2) is probably the one that would need the most insider knowledge, I'm not sure if the leaked NT sources go down to that level as I've never looked at them.
As for compatibility, 32 bit PowerPC Win32 binaries, 16 bit x86 Win16 binaries, and whatever x86 DOS stuff will run in an NT4 DOS box. No x86 Win32, only Alpha had an emulator for Win32 x86 stuff (until ARM stuff).
Or the author clean room reversed engineered the bootloader. Or there is enough information out there to forgo the need for any internal knowledge of Windows code.
The PPC code base was never explicitly targeted for Macs, but other systems from IBM/Motorola; but because it is a 'common' platform, the binaries themselves on the NT4 ISO do not need modification.
And yes, you can find the source code on GitHub. In multiple repos!
[0] https://www.neowin.net/news/exclusive-windows-2000--windows-...
A fun fact is that because of where he worked at the time was a DEC shop, the creator of PuTTY originally created it because he got an alpha windows machine and there was no compatible terminal emulator for alpha NT. He supported alpha builds well into the 2000s and IIRC only dropped it as the machine he was using still for builds finally died.