The Mac etc is what Jobs wanted. A sealed beige box that only did what Jobs envisioned it to do (produce a GUI and look good).
And yet variants of the Apple II kept outselling the Mac all the way until the II was unceremoniously dropped.
He was also responsible for NeXT. That company was about trying to make the power of Unix more mainstream.
Modern Macs are far more decended for NeXT computers than they are from the original baige box >= system 9 Macs.
This in much the same way as Linux use used just about everywhere these days, hiding behind a myriad of custom UIs.
When Jobs founded NeXT in 1985 there was no standard GUI layer for Unix. What you’re referring to as the “base layer” was all of Unix.
X was only started in 1984 I believe and Sun also has a competing incompatible alternative at the time too.
It wasn’t at all clear that the X Windowing System would win out as the dominant implementation.
Although that was mostly due to price, not expandability (the Mac II with all its slots and RAM expandability was introduced 6 years before the Apple IIe was discontinued)
That was mostly school purchases.