the real question i wonder is how do they even know what to write in code for building an emulator? are they ex employees that know the internals of xbox. how can you build an emulator for something you dont have access to (i assume all the software is proprietary)
Homebrew is important so you can slowly build up a battery of tests and compare to real hardware. In fact that's arguably more important than any insider knowledge because insiders have a remarkably incorrect view of the systems they build compared to people who spend years looking at it as a hobby. Insiders are ultimately very driven by the deadlines and pressures of shipping commercial systems and their knowledge only has to be good enough against all the other competing concerns.
It looks like a decade long game of trail and error.
I suppose you start at the basics, getting an interpreter purely for CPU code. So PPC to x86 code. Then onto more complex stuff like floating point VMX to x87. Once you have basic code processing, every time a line of code comes up that cannot be interpreted, you figure out what it does and then build the appropriate interpreter. Do this again and again and you would gain some basic function.
Basically just try running game code and keep plodding away like this until it works. Like giant bug hunt for 100,000 bugs. Very Tedious but necessary work. As for optimizing for performance, that is just black magic.
Judging from what i see people do on a couple of OG Xbox Discord rooms, i'd guess the biggest contributor would be reverse engineering the existing software (games, etc) using tools like Ghidra (and whatever was common for reverse engineering before it). After that you build a bunch of tests that you can try to run on the original hardware and compare with the emulator (e.g. this[0] repository contains a bunch of tests written against the open source nxdk SDK for the original Xbox - note that the effort goes back to very early 2000s and it is only in recent years that you could call it usable for more than basic things[1] - and the author often contributes tests to specific issues with the Xemu emulator[2] comparing it with the real Xbox).
[1] though nxdk/freexdk/openxdk had to "compete" with the leaked XDK that a lot of homebrewers used so there was much less incentive to contribute to the open source one
