What has Apple done to make macOS partially closed-source? The macOS operating system is on nearly every Mac. What has Apple done to make sure that their users cannot see, and thus recreate or modify, the low level code that makes macOS, macOS, how have they done it, and why can't we, the users, get around their method of hiding parts of macOS?
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)