Interestingly, Apple apparently builds Linux for their chips for hardware bringup.

Darwin is not *BSD?

It is, but I think he means "an open OS so it isn't just a paperweight anymore"

I believe Darwin is open source because most of it were bits of 3rd party open source software.

What frameworks and GUI Apple built on top of Darwin are not open source.

Also, releasing their software as open source goes against their business model as they wouldn't be able to enforce its use only on their hardware.

People wants Apple software but that only comes with their hardware.

Darwin source was release for a long time, with no restrictions on running it on Apple hardware. I’m not sure of the status of that now. They’re under no obligation to do so as it’s BSD licensed.

Yes exactly. Something that is FOSS and which one could tinker with.