> A lot of phone manufacturers "save on memory" and use the same memory chips for the baseband processor and the central cpu. Which means that it's a little bit cheaper ... and the baseband has access to all the phone memory and all peripherals connected through the memory bus (which is all of them in any recent phone).

This can be mitigated e.g. via an IOMMU: https://grapheneos.org/faq#baseband-isolation

> It may even be the case that these chips are integrated in the cpu (which I believe is the case for recent Apple chips).

I don't know whether it's true or not that they use the same RAM chips. But either way it doesn't change the fact that they can still be properly segregated via the IOMMU.