Can you explain? I vaguely recall using it on sparc workstations a couple decades ago, but I wasn't coding for that target. A quick wiki read says it was an API framework that was "endian free" and that it trageted the three architectures that next ran on. Was it doing emulation for software using it? Or was it, itself a single binary? Or both?

You had all binaries on the same executable.

https://en.m.wikipedia.org/wiki/Mach-O

And NeXTStep OpenSTEP's predecessor had already implemented it.

Might have been, I mentioned OpenSTEP, because NeXTSTEP didn't need to be cross platform, that requirement only came to be with the former.

NeXTStep was cross architecture I ran and compiled Objective-C code on HPPA, i386 and 68000 machines. I suspect I built for SPARC as well

I stand corrected.