The core of MacOS is Darwin which is open source. Based on the MACH/XNU kernel and *BSD. So any MacOS clone would start with using Darwin code.
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Emulating macOS at the Darwin layer is not necessary and often not advisable (see: GNUstep).
The goal was "source code compatibility with macOS applications".
Most Mac apps never call into the Darwin layer directly.
Lots of macOS applications need to drop to a few Darwin related features from time to time, if only to work around bugs in the frameworks. In addition, lots of applications run on macOS that use POSIX APIs, which are (typically) implemented in Darwin and not in the user-space frameworks.
But yet, I misunderstood the GP comment, and what I said was not really appropriate in that context.