> Linux is on every Android phone
Yes, and almost all consumer routers and quite a lot of other random pieces of hardware. Hence the billions of users.
How many of those pay for upstream maintainers?
> I don't have a clue what does Android actually use for filesystem drivers
"Yaffs" or "sparse" (which is just fancy ext2). https://source.android.com/devices/bootloader/images ; most Android kernels will also have exfat so they can use SD cards and USB sticks.
(I don't have a clue what does Android actually use for filesystem drivers, but I would _guess_ it's just Linux down there.)