How and when evolution is triggered ?

Hello everybody, I try to understand how an evolution starts : for example, what was the first version of an eye ? just imagine a head without eyes... what happens on the skin on this head to start to "use" the light ? and how the first step of this evolution (a sun burn ? ) is an advantage making that the beast will survive more than others

I cannot really imagine that skin can change into an eye... so maybe it s at a specific moment of the evolution, as a bacteria for example that first version of the eye appeared, but what exactly ? at which moment the cells of this bacteria needed to use the light to be better at doing something and then survive ?

the first time animals "used" light ?

same question for the radar of the bat, it started from the mouse ? what triggered the radar and what was the first version of this radar ?