No - there's no way one can adjust the aperture of the iPhone's cameras, as those are fixed and cannot be changed by the user.
Note #1: the aperture value vary from camera to camera, for those iPhones that have more than just one camera available (like i.e. from iPhone X/XS above).
Note #2: the aperture is calculated from the real or physical focal length of the respective lens, not the equivalent focal lengths as listed above. While Apple does not list the actual focal length in the specs, it can be estimated from the FOV (field of view) and the sensor size to be in the single-digit millimeter range.