The macOS kernel used since Mac OS X is XNU. Its name is commonly expanded as “X is Not Unix,” a playful reference to its NeXTSTEP origins and Unix-like design.
XNU began at NeXT and was adopted and expanded by Apple after its 1996 acquisition of NeXT. It combines a heavily modified Mach foundation with substantial BSD-derived components, including process management, networking, file systems, and Unix-compatible interfaces. Apple also developed IOKit as its kernel driver framework.
A common mix-up is Darwin. Darwin is the open-source operating-system core released by Apple; XNU is the kernel inside Darwin. Mach is another tempting answer because it supplies much of XNU’s underlying architecture, but Mach alone is not the complete macOS kernel. XNU remains the kernel family used across Apple platforms, including macOS, iOS, iPadOS, watchOS, tvOS, and visionOS.