It's very interesting to think AIX was ported to such a foreign architecture as it was so deeply tied to the R/S 6000 hardware. IBM Austin is where AIX was developed in the 90s.
RS/6000 was the fourth AIX platform after x86 (AIX 1.x), IBM RT/ROMP (AIX 2.x), and the mainframe (AIX/370, AIX/390)
How related to one another were those though? I remember AIX being expanded to “Any IBM UNIX”.
Not particularly. Any influence or overlap between them was in a strictly general sense. There are front end similarities but the internals are rather different, as would be expected for a rapidly evolving OS with lots of different authors. AIX didn't start to gel until around the 3.x or 4.x days.
AIX is still developed in Austin.
Define "developed".
They get the maintenance money of people still running apps on there while changing/fixing one thing or another a year.
Fixes are keeping up with CVEs fairly quickly. The process is actually pretty good.