Okay, so Apple made a PCB sandwich, but how does it work?

They created a third spacer PCB that lines the perimeter. And instead of connecting the two layers with a fussy flex cable, data travels along dozens of through-hole vias.

Here, the A11 SoC lies nestled at the center of the main board. You can get a sense of the 3D structure of the board from the X-ray photos. The cylinders around the edge are holes filled with solder connecting the two boards.