Apple has always shared the kernel of macOS after each major release. This kernel also runs on iOS devices as both macOS and iOS are built on the same foundation. This year, Apple also shared the most recent version of the kernel on GitHub. And you can also find ARM versions of the kernel for the first time.
The code was pushed to Apple’s open source site, as well as to their official GitHub mirror.
It’s important to remember that compared with Microsoft, Apple is way, way more open.
They open sourced their entire kernel (which is not required by the license of the BSD components). They lead the way for the ports of dtrace and ZFS out of Solaris, and they have fully open sourced their mDNSResponder implementation.
Edited 2017-10-02 22:36 UTC