The kernel at the heart of macOS is XNU.
XNU is a hybrid kernel originally developed at NeXT for NeXTSTEP and later adapted by Apple after acquiring NeXT in 1997. Its name is commonly expanded as “X is Not Unix,” a playful reference to its Unix-like heritage.
XNU combines a heavily modified Mach microkernel foundation with substantial BSD kernel code. Mach provides services such as low-level process management, virtual memory, and interprocess communication, while BSD contributes networking, file-system interfaces, Unix process behavior, and other core operating-system facilities. Apple’s IOKit framework supplies much of the hardware-driver architecture.
A common mix-up is Darwin. Darwin is the open-source Unix-like operating-system base that Apple publishes; XNU is Darwin’s kernel. macOS builds on Darwin but adds Apple’s proprietary frameworks, graphical systems, applications, and user experience. XNU also underpins Apple’s other operating systems, including iOS, iPadOS, watchOS, tvOS, and visionOS. Despite its name, macOS received official UNIX certification with Mac OS X Leopard in 2007.