The macOS Concept Journey — Darwin
Darwin is the core Unix based operating system used for macOS, iOS, watchOS, visionOS, iPadOS and bridgeOS — examples of the “uname” command output screenshots are shown below. It is based on code derived from BSD (OS distributed by the Computer Systems Research Group at the University of California, Berkeley) ,Mach (kernel developed at Carnegie Mellon University), NeXTSTEP (OS developed by NeXT Computer, which was acquired by Apple at 1996), other free open source projects and code developed by Apple (https://en.wikipedia.org/wiki/Darwin_(operating_system)).
Moreover, it is important to understand that Darwin does not include features like “Cocoa” (https://developer.apple.com/library/archive/documentation/MacOSX/Conceptual/OSX_Technology_Overview/CocoaApplicationLayer/CocoaApplicationLayer.html) or Aqua GUI (https://apple.fandom.com/wiki/Aqua). Due to that, software that is dependent on those (or others not included in Darwin) won’t run on Darwin (https://kb.iu.edu/d/akks). So, we can say Darwin is an independent operating system with a subset of features of OSes like macOS and iOS (https://developer.apple.com/library/archive/documentation/Porting/Conceptual/PortingUnix/background/background.html).
Also, Darwin is published under the license APSL (Apple Public Source License) version 2.0 (https://opensource.apple.com/apsl/). Thus, a few projects had been created with the goal of enhancing the operating system. The most known examples of those projects are: PureDarwin (https://github.com/PureDarwin/PureDarwin), OpenDarwin (which was shutdown on 2006, opendarwin.org).
By the way, Darwin currently supports different types of processors: Intel (x86/x64), 64-bit ARM, M1 and Raspberry Pi 3B. Old versions supported 32/64 bit PowerPC, 32 bit x86 and 32 bit ARM (https://en.wikipedia.org/wiki/Darwin_(operating_system)#Hardware_and_software_support).
Lastly, we can say Darwin is a Unix-like OS system core, which can be approximated as “Darwin= kernel + XNU + runtime” (https://zhangferry.com/2022/05/05/darwin_evolation/). XNU is the core/kernel part of the Darwin operating system (https://github.com/apple/darwin-xnu).
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