The basic idea is that some key bodily functions are regulated by semi-autonomous nervous systems located within the concerned organs. E.g. the heart has its own "chip", its own "microbrain" in the shape of a small pack of neurons that exite the various parts of the heart muscle in the exact sequence needed to make the heart go TA-tum TA-tum. Similarly, the digestive system has its own dedicated neural network, the enteric nervous system, embedded in the digestive system. Maybe that's the origin of expressions like "gut-feeling" or "to believe in one's heart that..." Our nervous system is much bigger than just our brain.
In evolutionary terms, our brain is just an unsually large concentration of neurons. The nervous system is always networked throughout the body, in all animal species, but more concentrated in smarter species.
Eg at the bottom of the evolutionary ladder, earth worms are made of rings and each ring is governed by its own micro-brain (called "ganglia" in the picture below), all connected by a neural chord. Their head's ganglia are only a bit bigger than the others (called "brain" in the pic below) because they manage more senses.
But there's no big mystery there because we're not talking of "minds" aka symbolic information management. Our bodily functions and their dedicated nervous systems do not need a representation of the world to work. It's pure machinery at that level. But our mind does
need to represent the world and itself in it. That's where the big mystery lies.