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PureDarwin
Darwin is the Open Source core of macOS, and PureDarwin is a community project to extend Darwin into a complete, usable operating system.
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InfluxDB
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However, you're again mistaken about Apple's own open source code. Most of their open source code is in relation to Unix/BSD stuff they've used in their operating systems, such as the Mach kernel and some (low-level, from my understanding) parts of FreeBSD. Over the years, they've slowly been adding their own proprietary code, and you'll find that even their open source macOS "base", Darwin, is far from complete (go have a look at the PureDarwin project to see more or less what can be made from the macOS code Apple has made publicly available).