I just tap them and move the slider. They both work for me.
Haven't even found anything at all related to aperture.
The aperture looking icon is actually the shutter speed control. A longer shutter time makes for brighter pictures, but can induce blur from camera shake. A shorter time gives crisper shots and can freeze action, but results in darker images if you don't have enough light.
There are no phones with actual aperture controls that I'm aware of. They all shoot "wide open", letting in the most light through the lens as possible.