What has Apple done to make macOS partially closed-source?
We don't know whether Apple did anything special here (like obfuscating their code or such).


A lot of software in macOS is actually Open Source and published on Apple Open Source
The source to the kernel (the core of the OS) is available from the above link but also on GitHub where it includes the version history.
You can also learn a lot about how everything works by reading through the documentation available (e.g. Mac Technology Overview)
And there is always the technical option to reverse engineer any library or application (even though it may be illegal to do so depending on where you live)