Alt text: meme with the 'Always has been' format Linux, MacOS, OpenBSD and ChromeOS logos on top of the Earth The first astronaut says 'Wait, it's all Unix?' A Windows logo, on top of the second astronaut. The second astronaut says 'Always has been' and points a gun to the first astronaut.
Dove into that some time ago.
NeXTSTEP was made by Steve Jobs after quitting Apple, awesome Software anr Unix based, but the hardware was overprices.
Then Mac bought NeXTSTEP back and made their first good MacOS on the Unix base, which is what they use to this day.
Afaik they also use the same Kernel and and some more in all devices.