A DSLR lens still controls aperture with the iris blades inside the lens, just like older manual SLR lenses. The lens is normally held wide open so the viewfinder stays bright and focusing is easier.

When you press the shutter, the camera actuates the lens mechanism and the aperture is stopped down to the selected f-number for the exposure, then reopens immediately afterward. On modern systems this actuation is very fast and is often driven electronically, so it’s easy to miss with the naked eye.

So the aperture is not changing in some hidden digital way—the camera is still physically closing the iris blades in the lens, just very quickly at the moment of exposure.