Exposure is measured on on, and depends on three different things. ISO is one, this affects the smoothness of the image, and the pixelation of it. The aperture affects the amount of time the lens stays open, and time/light gets in. Shutter speed is the how fast the lens opens and closes.
Question 1- A stop is the measured amount of light a camera takes in, like the exposure.
Question 2- smaller aperture the larger depth of field, and larger aperture, the smaller depth of field.
Question 3- the shutter speed affects the amount of light that gets in the picture, as well as the blurriness or clearness of the image. The focal length affects this because of the focus in the image.
Question 4- to use a shallow depth of field, take macro photographic pictures.
Above is Robert Clarks photograph which has a high iso, because of the clarity and brightness in the image.
My emulation uses a high iso to show the clarity of the building in the back.