The opening of a lens is inherently an aperture. However, cell phones don’t have variable apertures. You won’t find them in small point and shoot cameras, either.
The issue here is diffraction. When light passes through an aperture, it bends just a little… the smaller the aperture, the more the bending. So point your camera at a perfect one dimensional point of light, and you’ll get not a focused point of light, but a disc, called an Airy disc, after George Airy, the guy who first worked out the mathematics of light diffraction.
In most cellphone cameras, that disc is just a little larger than your pixel size, based on the aperture they chose, usually around f/1.8 in a premium smartphone, f/2.2 in an economy smartphone. So a little bleeding of light from pixel to pixel is no big deal, but too much of that and you’re losing sharpness. Things are so tight on a smartphone, even a little stopping down of an aperture would reduce the image sharpness. So you don’t get a variable aperture. See more here: Pixel Size, Aperture and Airy Disks.
Smart phones, just like small point and shoot cameras, do not have mechanical shutters. They use electronic shutters. In a CMOS image sensor, the “electronic shutter” is really just a process. Part of the sensor is turned on — made sensitive to light — the light is accumulated in the charge wells for each pixel, and then the sensitivity is disabled and the charge at each pixel read out. This basically “rolls” down the sensor, each part of the sensor getting the same exposure, but not precisely at the same time.
This same basic process is used on camcorders and it’s usually available on mirrorless professional cameras, sometimes for extra speed. For example, my Olympus OM-D E-M5 mark II has a top mechanical shutter speed of 1/8000th second. But using the electronic shutter, I can go to 1/16,000th second. And it’s perfectly silent.
