Apple Darwin is an open-source operating system that is designed and released by Apple. Based around multiple other open-source operating systems, Darwin is a Unix-like with code derived from NeXT’s NeXTSTEP, as well as BSD and Mach, among others. Darwin makes up the base of almost all of Apple’s operating systems including macOS, iOS, watchOS, tvOS, iPadOS, as well as the less commonly known operating systems such as bridgeOS and audioOS.
Darwin is, at the time of writing, still actively updated and new versions are released by Apple corresponding to each iOS and macOS update. Darwin currently supports many of Apple’s recent hardware processor architectures such as the current Intel x86 as well as the ARM processors used in iOS devices. It is also set to support Apple’s in-house desktop-class ARM-based processors called Apple Silicon. Support for older processor architectures, such as PowerPC, have been discontinued.
While the core fundamentals parts of Apple’s operating systems remain open source with Darwin, the infamous “Aqua” GUI, as well as core APIs such as Carbon and Cocoa, as well as the Quartz Compositor remain closed-source.