Im interested to know the ideas behind downforce and how it aids in performance on cars.

Im led to believe that the lower the pressure under the car and the higher pressure above the car results in downforce.

What kind of systems make this acheivable?
Wings will cause downforce obviously, but how much does the flat smooth floorplan aid in this process ?

What speeds does the smooth underside come into its own and aid in grip/downforce?

30mph+
80mph+
120mph+

Hope any one can help or offer there knowledge in this HUGE topic.