The A/UX Toolbox PDFs cover a lot of interesting technical details for the curious:
http://bitsavers.org/pdf/apple/mac/a_ux/aux_2.0/030-0787-A_A..."/mac/sys — This directory contains the system folders for startup and login. The System file provided with Release 2.0 of A/UX is almost identical in functionality to the System file provided with Release 6.0.5 of the Macintosh system software."
Additionally I always thought the Gestalt Manager was a System 7 thing and was surprised to learn it was introduced along with system 6.0.4 and A/UX so applications could tell if they were running on it: https://developer.apple.com/library/archive/documentation/ma...