They're embedded. The main phone has no access to them apart from the hardware interface that they provide.
Yes, the other ARMs are low power, and that's ideal for the networking parts which are active more often than the CPU (e.g., in your pocket the baseband has to listen for incoming texts and phone calls).
You can't run iPhone application software on them. You can upgrade their embedded firmware via software update. They're a hardware function partially written in software running on an embedded CPU so that it can be updated. That's how Draft-N wireless gets updated to Full-N eventually.
thanks buddy!