> Improving FreeBSD will make it easier to run BSD on non-apple hardware which will eat into their bottom line.

The number of people who want to run FreeBSD on their laptops probably numbers in the hundreds. Not exactly a threat to Apple's bottom line.

On the other hand, some of those people are FreeBSD developers who create and maintain code that Apple would like to have the option of using. That relationship is worth something to Apple.

There's no GPL in the BSD sources used by Apple or Sony. They are free to release their operating systems as closed source; Sony does this. Apple releases Darwin sources "out of the goodness of their hearts", meaning, back in the 2000s they wanted to capture mindshare amongst the tech community for whom Linux was the strongest contender. Now that the future has refused to change, the year of the Linux desktop never materialized, and macOS has become the default developer's workstation OS, Apple has been much more sparing with Darwin source drops and may cease them altogether.

Why would BSD use GPL?

BSD has a BSD license. It doesn't require source code releases.

> MacOS was absolutely derived from BSD through NeXTSTEP.

The OS-X (now branded as "macOS") kernel was not, and is not, a derivative of the FreeBSD kernel, or any other BSD, even though macOS/OS-X has a FreeBSD kernel component due to its Mach heritage. The userland tools are however BSD. OS-X's kernel is XNU and from the XNU GitHub repo[0]:

  XNU kernel is part of the Darwin operating system for use 
  in macOS and iOS operating systems. XNU is an acronym for X 
  is Not Unix. XNU is a hybrid kernel combining the Mach 
  kernel developed at Carnegie Mellon University with 
  components from FreeBSD and a C++ API for writing drivers 
  called IOKit. XNU runs on x86_64 and ARM64 for both single 
  processor and multi-processor configurations.

EDIT:

In theory, XNU could simultaneously run the existing FreeBSD subsystem alongside Linux and/or MS-Windows ones. In practice, this would be a herculean effort fraught with difficulty.

See QNX[2] for another example of a micro-kernel OS architecture.

0 - https://github.com/apple-oss-distributions/xnu

1 - https://books.apple.com/us/book/mac-os-x-internals/id4343583...

2 - https://en.wikipedia.org/wiki/QNX

The first version of MacOS X Server was based on an unreleased version of NEXTSTEP which in turn used 4.4BSD and Mach 2.5. Around BBB 1997-1998 a lot of userland was synced with bits from not just FreeBSD but the other BSD distributions, if my memory serves me correctly. MacOS X moved to Mach 3. That’s a very very long time ago though, and Apple obviously did a _lot_ of their own CoreOS engineering, things like launchd and XPC don’t have FreeBSD equivalents.

But hey, Darwin is open source so if someone wants to do go on a provenance archeological dig, it could be done!

Large parts have been rewritten: they very different and don't show any BSD heritage.