I haven't looked at apple specifically, but generally the approach taken with HBM on GPUs etc is to make multiple chiplets and connect them together very close and with very high bandwidth. The chiplets are made with different processes, optimised for each specific use case.

There are a few different variation, such as having a carrier chip below (as an interconnect), having a small PCB that everything is mounted to, or even vertically stacking chips (AMD's 3D v-cache does this on some of their CPU models).