http://toastytech.com/guis/rhap.html
And there's some description of the emulation nanokernel:
https://en.wikipedia.org/wiki/Mac_OS_nanokernel
But in its way OS X was much more of a clean break: a whole new OS, ported to the old hardware, tweaked to understand filesystems and file formats and given a shallow cosmetic resemblance.
A/UX was a much tighter integrated system. Some of the OS functionality was provided by actual classic MacOS.
What was left, if that was removed, was a fairly unremarkable Unix for its time. You could even boot it in a sort of vanilla-Unix form.
You can compare and contrast here:
https://www.aux-penelope.com/screenshots.htm
E.g. for v3, top 2 rows (pics 1-6) are in Mac mode, bottom row (7-9) are in Unix mode. A bit like the ANS with AIX, there's little to appeal to Apple users here:
I asked this myself.
The thing that I didn't think about was that A/UX combined 680x0 UNIX™ code with 680x0 MacOS code in very clever ways. MacOS provided the windowing, terminal emulators, the filesystem browser, etc. Unix provided the underlying kernel, the filesystem, networking, etc.
(This is a hand-wavey simplification for illustrative purposes.)
The point being, intimately intertwined, closely integrated.
But classic MacOS only ran on 680x0 and to port it to PowerPC, Apple invented a clever hack built around partial emulation: a nanokernel containing a 680x0-to-PowerPC interpreter (with a way for 680x0 code to call PowerPC code), on top of which ran the MacOS Toolbox ROM image, on top of which ran most of MacOS in 680x0 code. Then they profiled that, and converted the most performance-critical parts -- and only those -- to PowerPC code.
When I ran a PowerMac with classic MacOS, I had a little indicator applet in the menu bar. It was red when executing 680x0 code and turned green for PowerPC code. (Not very helpful for colour blind folks, but hey, fine for me.)
It was red almost all the time. Only years later when substantial PowerPC-native apps started to appear did it sometimes stay green long enough to see.
It wouldn't have been so hard to port the Unix bits of A/UX to PowerPC-native, but that would have ruined the integration between the Unix code and the MacOS code. Getting MacOS all running natively would have been a massive task. Apple never did it; it replaced the entire OS with NeXTstep.
Getting 680x0 MacOS code integrating with PowerPC A/UX code would have been a major technical challenge.
It sounded easy and obvious but it was an even bigger project than Copland, and Apple wisely dediced against it.