In our daily life, we always use many foam-related products, such as foam hand sanitizers,shower gel, facial cleanser, etc. Why can the liquid in the bottle become foam and be squeezed out?

Because when the water and the liquid containing surfactant are mixed, a certain negative pressure is formed around the liquid column after being ejected at extreme speed, and the negative pressure is generated to attract the gas into the mixing part, so that the air and the liquid are mixed again, and the foam is formed from the pump. 
So what are surfactants?Any substance that dissolves in water and can significantly reduce the surface energy of water is called a surface active agent or surface active substance.
Common surfactants are composed of two parts, one part is a hydrophilic (oleophobic) head group, and the other part is a lipophilic (hydrophobic) carbon chain. When cleaning the surface, the lipophilic carbon chain is combined with oily substances,and then under water rinsing, the hydrophilic head group is combined with water, it can to wash off the stains.