Darwin itself was originally released by Apple in November 2000. It is a fork of Rhapsody, which was the codename used for Apple's next-generation operating system after the purchase of NeXT in 1998. Darwin utilises the XNU kernel, and currently runs on modern x86-64 processors, as well as 32-bit ARM processors in the case of older iOS devices (e.g. the iPhone 5C).
Many well-known elements of macOS such as the Cocoa framework and the famous Aqua graphical user interface are not included in Darwin, and unfortunately remain closed source. — A Look at PureDarwin