Quoting Jason White (jason@jasonjgw.net):
Apple didn't have a kernel capable of preemptive multi-tasking, as I recall, so they developed a derivative of 4.4 BSD and then used technologies from NextStep for the GUI.
Actually, the kernel _is_ NeXTStep's xnu kernel, which is 4.3BSD upper layers melded to a Carnegie Mellon Mach 3.0 microkernel (initially v. 2.5). https://en.wikipedia.org/wiki/Xnu Apple didn't develop xnu. NeXT did. You're correct about the GUI, the only really major change from NeXTStep's imaging system being Apple changing it over to Display PDF instead of Display PostScript -- solely because Adobe makes PDF available royalty-free, whereas every NeXT system shipped with Display PostScript required a payout of patent royalties. (Other changes: jettisoning the crufty printing subsystem and replacing it with CUPS, jettisoning the odd Netinfo directory service originated at Sun Microsystems and never really adopted anywhere but NeXT and replacing it with one based on OpenLDAP.)