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.
https://www.youtube.com/watch?v=36myc8wQhLo
Roscoe's talk is fairly long (the video is > 1h), but the basic thesis was taken up in a segment of Bryan Cantrill's 20 min OSFF talk:
https://www.youtube.com/watch?v=XbBzSSvT_P0
Both talks are very good; I recommend watching both, in either order.