"The module uses light emitting diodes (LEDs) in conjunction with a light sensitive sensor to measure the absorption of red and infrared light by the user’s fingertips. "
A bit deeper, the PPG (photoplethysmogram) signal is measured by the user holding sensors on the phone with both hands. Using math I don't understand, the chip can then calculate a fairly accurate (within a couple mmHg) blood pressure. There are a couple papers describing the method on the first page of this search:
see https://www.mediatek.com/features/sensio-biosensor-module