How the jack exerts a force?

Answer 1

A vehicle jack exerts at least two forces, one up on the vehicle equal to the vehicle weight and one down on the ground equal to the vehicle weight plus the jack weight. Of course gravity exerts a force equal to the vehicle weight so that the net force on the jack is zero. You increase the force up on the vehicle and down on the ground with a lever or screw that makes the jack longer.

Answer 2

Or, besides lever or screw, one can use hydraulic force (see Pascal's principle)