As it has done in the past, Apple has released the open source Darwin code for macOS 10.12 Sierra. The code, located on Apple’s open source website, can be accessed via direct link now, although it doesn’t yet appear on the site’s home page.
The release builds on a long-standing library of open source code that dates all the way back to OS X 10.0. There, you’ll also find the Open Source Reference Library, developer tools, along with iOS and OS X Server resources.
The lowest layers of macOS, including the kernel, BSD portions, and drivers are based mainly on open source technologies, collectively called Darwin. As such, Apple provides download links to the latest versions of these technologies for the open source community to learn and to use.
Apple usually updates its open source Darwin code for public consumption not long after the public release of its latest update to OS X. Although the recently released macOS Sierra is the first version to adopt a new name, Apple has maintained the open source tradition with this release. You can read more about Apple’s history of open sourcing here.