https://www.theregister.co.uk/2017/11/03/iphonex_ifixit_teardown/

The iPhone X's face recognition may be experiencing teething problems but the thousand-quid handset is a masterpiece of engineering.

Apple is the first to market with new, dense circuit board design called Stacked SLP, often referred to misleadingly as a "stacked logic board". Today's phones use 10 layers of copper on the PCB. Stacked SLP uses 20. This permits for a higher density of components in a given surface area. The iPhone X had also been tipped to have a 10-layer AP board, eight-layer RF board and two layers of interposers.