Glossary
Darwin
The open-source Unix-like core under macOS, iOS, watchOS, tvOS, and visionOS. XNU kernel + BSD userland + Apple's frameworks-free base system.
Darwin is the open-source operating system under macOS — the XNU kernel plus a BSD-derived userland (bash/zsh, coreutils, libc) plus Apple-specific infrastructure (launchd, dyld, IOKit).
Darwin is not by itself macOS. macOS adds the closed-source layers on top: Cocoa/UIKit/SwiftUI, Metal, Core Image, the entire app framework stack. Darwin is the headless OS underneath that you could, in principle, run on bare hardware (and historically people did, via PureDarwin and OpenDarwin).
Most of Darwin is published as open source at github.com/apple-oss-distributions. The notable closed pieces:
- The APFS driver (the format is documented; the implementation isn't open).
- The Sandbox.kext policy engine.
- The Bluetooth and Wi-Fi stacks.
Everything else — XNU, Libc, libpthread, libdispatch, dyld, launchd, Security framework, the bsd userland tools — is in the open repo and refreshed each macOS release.