Android and ChromeOS are more Linux than MacOS and iOS are a FreeBSD. Apple closed the source quite a while ago and they have their own fork which I bet is very far away from the original now.
FreeBSD is licenced under BSD licence, they don't have to do shit and they don't. Windows is also using a lot of BSD code and you'll never see the sources.
You can say the same about every distro. There's literally no difference between Android and Ubuntu from this perspective. Also that's how a lot of code feeds back into the kernel - it gets tested in the wild and if it's good and useful it gets PRd and merged back.