Essentially, using appropriate image analysis, they can sense the pulse of blood moving through the skin
The other comments provided the academic sources, but the Cardiio app on my iPhone accurately takes my heart rate from my face with the selfie camera and that was released in 2012 I think.
This video from 2013 references to possible methods: imperceptible changes in skin color and imperceptible movement of the head. https://youtu.be/EhZXDgG9oSk