^ This. Prior to Apple, the phone OEMs and carriers had hooks all into your baseband firmware for all kinds of things; firmware updates, CALEA hooks, automatic provisioning, etc...
Source - used to certify these things in a lab environment.
That's great to know. I'd like to feel reassured but I'm sure there's new exploits somewhere, the carrier level i think is just so attractive to large scale actors. Any internet links to read up on this, and on the next generation of possible carrier level exploit vectors?
This is very outdated, at least for a significant number of smartphones (including all iPhones, but not limited just to those). Apple and IIRC other manufacturers long since isolated the baseband, treating it simply as a standard USB or PCIe peripheral (and in the latter case using an IOMMU with it amongst other things). It has zero special access to anything on the rest of the phone which in the smart phone era is where everything of interest actually lives and happens.