There's a difference between "what it does" and "what it can do" however. I mean, whatever well-defined interface is used, hardware design may leave other options open and unused.
Kind of like vmware provides nice interface for folder sharing, but in practice can just write directly to whatever files/memory they want.
It all depends on particular design. There are even some out there where baseband OS and Linux run under the same proprietary hypervisor. On modern phones it's pretty rare to be able to say with 100% certainty that the baseband OS doesn't have any access to user memory - and often you can say with 100% certainty that it can access other interesting stuff, like GPS or microphone, without any supervision from the user.
Kind of like vmware provides nice interface for folder sharing, but in practice can just write directly to whatever files/memory they want.