Electrons have the same charge, so their abundance and scarcity become the overriding factors in their tendency to move in predictable ways. In fact, electronics is the practice of controlling electron balances, to get the little rascals to reliably do tricks, while also trying to avoid getting ourselves rebalanced by careless practitioning .
I've seen similar circuits that use a lightly loaded zener diode to generate the random noise.
Same principle.
The LED will conduct reverse biased when hit by photons, much as any photo-diode.
A layperson might ask, "why would electrons move differently when electric field affects them all since all electrons have same charge?"
Electrons have the same charge, so their abundance and scarcity become the overriding factors in their tendency to move in predictable ways. In fact, electronics is the practice of controlling electron balances, to get the little rascals to reliably do tricks, while also trying to avoid getting ourselves rebalanced by careless practitioning .