The graphics and the baseband functions are not handled by the same chip. There is a chip for every feature. For example, in many phones and devices, the graphics chip is provided by nVidia, Mali, Adreno, etc., while the baseband functions are provided by the RTOS (real-time operating system). These chips usually have their own firmware and RAM.
I recommend reading on Wikipedia regarding the baseband functions: https://en.wikipedia.org/wiki/Baseband_processor
