principle about SPAD for ToF ranging module from ST
When a single photon hits the photodiode (and is converted), it will avalanche, making a big signal that the logic can see, very quickly.

It will then be blind to subsequent photons. 

This is OK, the chip is going to average a huge number of repeats. 

These chips arrived on sale years after there were prototypes, I suspect it was a harder trick to actually get working.