As you extend the bellows, the aperture stays the same. The aperture is related to the focal length and the dimeter of the lens. These do not change.
What changes is the amount of light that falls on the area of the format. As the lens moves away from the focal plane, the image circle expands, which spreads the available light out. The intensity of the light falling on the area of the format (the film) drops off inversely with the square of the distance.
See
https://en.wikipedia.org/wiki/Inverse-square_law for an explanation of the inverse square law.
Look at
https://en.wikipedia.org/wiki/F-number for an discussion of f-stops.
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