Darwin doesn't even remotely resemble MacOS. It's the kernel and open-sourced user-land components. It resembles(but is not at all) a very old FreeBSD-ish system if loaded.
Just out of curiosity, what else if not the kernel would be interesting to copy from MacOS? I use a Mac but just don't have the inclination to learn about it under the hood (unlike Linux). Besides the various apps they include and the graphical / window system, I can think of LaunchD, drivers, but not much more that would be worth a lot to China.
Good high-level mental mode, but it's not a clean separation like that.
Think Open Source Android vs. Google's Android, or Chromium vs. Chrome. It's not just dragging and dropping some UI libraries on top. There's a ton of glue, and sticking shims into non-obvious places that make a lot of the magic happen.