Smartphone touch screens are capacitive which do not use the pressure of the finger to create a change in the flow of electricity. Instead, they work with anything that holds an electrical charge including human skin. (Yes, we are comprised of atoms with positive and negative charges.)
Capacitive touch screens are constructed from materials like copper or indium tin oxide that store electrical charges in an electrostatic grid of tiny wires, each smaller than a human hair.
There are two main types of capacitive touch screens, surface and projective. Surface capacitive uses sensors at the corners and a thin evenly distributed film across the surface whereas projective capacitive uses a grid of rows and columns with a separate chip for sensing. In both instances, when a finger hits the screen, a tiny electrical charge is transferred to the finger to complete the circuit, creating a voltage drop on that point of the screen.
This is why capacitive screens don’t work when you wear gloves; cloth does not conduct electricity, unless it is fitted with conductive thread.