what type of os is darwin, the core of os x?
It is composed of code developed by Apple, as well as code derived from NeXTSTEP, BSD, and other free software projects. Darwin forms the core set of components upon which macOS (previously OS X and Mac OS X), iOS, watchOS, and tvOS are based. It is mostly POSIX-compatible, but has never, by itself, been certified as compatible with any version of POSIX.