What has Apple done to make macOS partially closed-source? [closed]

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)