What open-source Unix-like operating system forms the base of macOS? Darwin. It is the open-source core on which macOS is built, providing fundamental Unix-like services beneath Apple's graphical software layers.
Darwin combines technology from several sources. Its kernel, XNU, blends Mach with substantial BSD-derived components, while Apple's work added system services, drivers, and other infrastructure. Apple released Darwin source code under the Apple Public Source License beginning around 2000.
Darwin is not identical to the complete macOS product. macOS adds proprietary frameworks and user-facing technologies such as Cocoa, Carbon, Quartz, and the Aqua interface. This is why a standalone Darwin installation cannot simply run ordinary Mac applications.
The same Darwin foundation also supports Apple's other major operating systems, including iOS, iPadOS, watchOS, tvOS, and visionOS. A common mix-up is naming FreeBSD: Darwin uses important BSD-derived code, but Darwin itself is Apple's distinct operating-system core.