There was a great talk I can't find right now about how the share of computer hardware that it's directly controlled by the OS kernel keeps on shrinking.
In a modern smartphone, the OS directly controls just one of a dozen chips. The rest are running their own OSes which do their own thing independently of the main OS.
I'm guessing it's Tim Roscoe's keynote on how most fundamental questions about what the hardware is actually doing are invisible to traditional OS abstractions:
In a modern smartphone, the OS directly controls just one of a dozen chips. The rest are running their own OSes which do their own thing independently of the main OS.