BSD UNIX has a longer and more complicated history than Linux. It started in 1978 as a derivative of AT&T's UNIX. Releases from the University of California at Berkeley (UCB) came in source and binary from, but they were not open source because alicense from AT&T was required. BSD UNIX's development was slowed by a lawsuit by AT&T, but eventually a fully functional, open-source version, 4.4BSD-lite, was released in 1994.
Just as with Linux, there are many distributions of BSD UNIX, including FreeBSD, NetBSD, OpenBSD, and DragonflyBSD. To explore the source code of ReeBSD, simply download the virtual machine image of the version of interest and boot it within Virtualbox, as described above for Linux. The source code comes with the distribution and is stored in /usr/src/. The kernel source code is in /usr/src/sys. For example, to examine the virtual memory implementation code in the FreeBSD kernel, see the files in /usr/src/sys/vm. Alternatively, you can simple view the source code online at https://svnweb.freebsd.org.
As with amnyu open-source projects, this source code is contained in and controlled by a version control system-in this case, "subversion" (https://subversion.apache.org/source-code). Version control systems allow a user to "pull" an entire source code tree to his computer and "push" any changes back into the repository for others to then pull. These systems also provide other features, including an entire history of each file and a conflict resolution feature in case the same file is changed concurrently. Another version control system is git, which is used for GNU/Linux, as well as other programs (https://www.git-scm.com).
Darwin, the core kernel component of macOS, is based on BSD UNIX and is open-sourced as well. That source code is available from http://www.opensource.apple.com/. Every macOS release has its open-source components posted at that site. The name of the package that contains the kernel begins with "xnu". Apple also provides extensive developer tools, documentation, and support at http://developer.apple.com.
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