And in what way is the Linux kernel the "fundamental principle" or "foundation" of Android? How is it so important that you can rip it out and replace with something totally different without much change to the applications running on top?
You can replace NT with Linux (or Linux with NT) as well. Userspace has nothing to do with this, kernel has its API and the userspace is built on top of it
> You can replace NT with Linux (or Linux with NT) as well.
No, not quite, not without enormous effort. Look at Cygwin or WSL or Wine:
tons of work poured into and still not there yet.
On the other hand, typical Android application only ever touches Android's
API and doesn't even leave its JVM. Replacing whole kernel (with obvious
single point of work in Java interpreter) would be practically invisible.