iPhone cameras lenses do not have movable iris so the Aperture isn’t adjustable. Instead, Apple provides different lenses with combinations of Aperture and Focal Length. For example, the iPhone 13 Pro has:
#TODO https://www.dpreview.com/articles/6780391159/all-apple-iphone-13-and-13-pro-camera-upgrades-explained #TODO https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care
Portrait Mode works better with the 2x lens and it allows the adjustment of f-stop. It takes a bit of work, so it’s generally easier to just take a shot and fiddle with depth later.