Flipping the digitizer on its back reveals the most significant change thus far from the lookalike iPad Air: the addition of Touch ID, and its associated hardware.

NXP 8461A1—also visible as a 5x4 ball grid array hiding under the speaker in this sweet X-ray image.

At least this lovely button is still a button (unlike the solid-state un-button that debuted on 7-series iPhones). Peep that switch under the Lightning port traces.

Likely hall sensors