"I'd just like to interject for a moment. What you're referring to as macOS, is in fact, macOS/Darwin, or as I've recently taken to calling it, macOS plus Darwin."

"What you're referring to as Darwin, is in fact, Darwin/XNU."

"What you're referring to as XNU, is in fact, BSD/Mach."

I seem to remember it being possible to run macOS-less Darwin several years ago, not sure if that's still possible or if Apple has modified it so much at this point that it's useless without at least some macOS components.