Why is MacOS closed source if it is a Linux based OS?

Just to clarify, both macOS and Linux have UNIX elements. That doesn't mean macOS is Linux based.

For one, it's not Linux based.

The Darwin kernel, which contains the UNIX-like foundation, and some other components are open-source.

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Apple does a lot for the open source community and has contributed a lot. Many people don't know about: https://opensource.apple.com

Also, to my knowledge, not all *nix based OS's are fully open source.