Check out Is Android really just Linux? on Android Authority. It’s a considered, logical walk around the issues involved.
A straight forward question, it seems, but it all depends…on the meaning of your terms…
Jessica Thornsby covers how you would define Linux, then how you would define a Linux distro, and then the issue of different Androids – the Android Open Source Project (AOSP) and the role of Google Mobile Services (GMS).
She then considers the points both in favour and against considering Android as Linux.
There’s really only one argument in favor of classifying Android as Linux, but it’s a strong one: every Android smartphone or tablet contains a Linux kernel. You can even see what version of the kernel is installed on your device, by opening its ‘Settings’ app and navigating to ‘About Device > Software info.’
However, Android isn’t just the Linux kernel; it’s a fully-functioning operating system that features lots of additional libraries, a GUI, apps and much more, so when we ask “is Android Linux,” we’re actually asking “is Android a Linux distro?”
…And where does GNU or TouchWiz fit in? You’ll see.
What would I say? For what it’s worth, I agree with the commenters (do check out the Comments section) who opt for the formulation “Android is a Linux-based operating system”…