Darwin is the open-source core of MacOS. It is a whole operating system but doesn’t include everything that MacOS does, mainly it doesn’t provide the Quartz Compositor. While it is possible to install X11 onto Darwin, GUI applications that were compiled for Quartz will not work under X11. CLI’s however should work seamlessly.
Darwin consists of:
- Mach 3.0
- operating-system services based on 4.4BSD
- high-performance networking facilities
- support for multiple integrated file systems
This means that Darwin can be compared to a GUI-free operating system like Arch Linux and MacOS can be considered as one that builds on top of the GUI-free core but provides a GUI, like CachyOS or Manjaro Linux.
The sources for Darwin are available at opensource.apple.com/releases.