The term “decisive moment” is a concept used by photojournalists. “The decisive moment refers to capturing an event that is ephemeral and spontaneous, where the image represents the essence of the event itself.” As the photographer, you must be ready some elements would be Figure to ground, Repetitive theme, and Diagonals/Golden Triangle. These elements are key to obtaining the best photograph.
Bresson shot many photos to capture the ephemeral. He was ready because he was photographing. The elements come together while you are shooting.They came together because he was working the subject , using the camera frame and composition while watching the light. Not all his photos were successful, but they led to the successful ones.
So shoot a lot, and learn from the good and not so good images.