I don’t understand why Google didn’t just made Android closed source. They would’ve made custom roms impossible and therefore more able to control and spy on it’s users. Why did Google made Android open source, while Microsoft’s Windows is still closed source to this day?
Kernel is irrelevant. By the way, MacOS kernel is open source and is derived from one of BSDs.
MacOS is derived from Darwin but heavily modified and now closed source. While GPL is copy left, BSD has no such requirement so you can effectively make your derivative OS closed source, which NeXT and subsequently Apple did.
Yes, Darwin got closed, but it was open for a very long time. Yet MacOS remained closed and proprietary. Again - kernel and its licence doesn’t matter.