NeXTSTEP appears to have first gotten incorporated throughly into the OS X codebase. Browse through the Foundation library for the Mac - https://developer.apple.com/documentation/foundation/ . Everything that starts with NS was part of NextStep.
OSX/macOS/iOS is the latest evolution of NeXTStep/Mach which originated in the Aleph (and other) academic kernels.
(of course OS's evolve pretty far in a few decades...)
It's wrong.
Original NeXT classes were prefixed NX_. Then NeXT worked with Sun to make a portable version of the GUI that could run on top of other OSes -- primarily targeting Solaris, of course, but also Windows NT.
That was called OpenStep and it is the source of classes with the prefix NS_ -- standing for Next/Sun.
https://en.wikipedia.org/wiki/OpenStep#History
This is why Sun bought NeXT software vendor Lighthouse, whose CEO Jonathan Schwartz who later became Sun's CEO.
Unfortunately for NeXT (and ultimately for Sun), right after this, Sun changed course and backed Java instead.
Not quite. Everything in Foundation gets the NS prefix because it's in Foundation; only a fraction of it came directly from NeXT.