Back to those capacitative sensors we mentioned. Under that funky film is certainly a grid of...something.

Each of the gold rectangles on this back panel is a single plate of a parallel plate capacitor.

Each box has a tiny trace (the dark areas are actually highways of many traces) running back to the control chip. Which...

...We find on the backside of the panel, probably. This Apple-custom chip is responsible for wrangling the capacitors—keeping them charged and measuring changes in current to and from each.