It comes from no particular part of the aperture, rather it comes from all over the aperture since it is the interference of all the wave components that results in the diffraction pattern
I don't feel comfortable to dispute a physicist's statement regarding physics, but I looked it up in the McGraw Enciylopedia of Science, which cites from "Fundamentals of Optics" by White, "Introduction to Classical and Model Optics" by Meyer-Arendt and "Lectures on Theoretical Physics" by Sommerfeld. I don't know from which source following paragraph comes:
The bending of light, or other waves, into the region
of the geometrical shadow of an obstacle. More exactly, diffraction refers to any redistribution in space of the intensity of waves that
results from the presence of an object that causes variation of either the amplitude or phase of the waves.
Specifically related to the Fraunhofer diffraction, it says
... At the instant that the incident plane wave occupies the plane of diffracting screen, it may be regarded as sending out,
from each element of its surface, a multitude of secondary waves, the joint effect of which is to be evaluated in the focal plane of the lens.
My questions are: if I accept, that not the edge of the opening causes the diffraction but the light waves cause it on their own, then
a. how does this depend on the size of opening?
b. why does the opening exactly at the location of the aperture count, not for example the diameter of the first lens element?
c. this is expanding on b: at which point of the lens (at which point of the travel of the light rays) does the diffraction start?
I do understand, that this is far away from everyday's photography, but perhaps I am not the only one intrigued by the notion, that the presence and shape of the aperture is not the cause of diffraction.