osx was nextstep with os9 stuff (toolbox etc) ported over as carbon (with a lot of other os9 stuff) to support porting older apps [0]
[0] https://en.wikipedia.org/wiki/Carbon_(API)
but the kernel, gui, drivers everything was descended from nextstep
edit: (next/openstep)
The Objective-C drivers framework (DriverKit) was replaced by a C++ one based on a language subset (IO Kit).
It is no accident that IO Kit replacement is named Driver Kit as homage to the original NeXTSTEP driver framework.
osx was nextstep with os9 stuff (toolbox etc) ported over as carbon (with a lot of other os9 stuff) to support porting older apps [0]
[0] https://en.wikipedia.org/wiki/Carbon_(API)
but the kernel, gui, drivers everything was descended from nextstep
edit: (next/openstep)