there is no interference if the slit is infinitesimal.
But in this regard infinitesimal is whatswhat's smaller thenthan a wavelength (maybe half wave lengthwavelength as iI don't remember the exact solution).
But if it is larger thenthan that you can say that it is composed of several infinitesimal slits next to each other with 0 distance between them. youYou do get interference.
I think you can understand the second part if, if you look at the distance between the dark parts. theyThey have the same width and their distancedistances from the middle are the same