Package-on-Package has been used in mobile SoCs for a long time. This wasn't an Apple invention. It's not new, either. It's been this way for 10+ years. Even cheap Raspberry Pi models have used package-on-package memory.
The memory bandwidth of flagship iPhone models is similar to the memory bandwidth of flagship Android phones.
There's nothing uniquely Apple in this. This is just how mobile SoCs have been designed for a long time.