Why do you use concave mirrors on the passenger side?

Actually it is a convex mirror, it bulges out in the center. This increases the field of view from a normal flat mirror so that you can see more area from side to side. So that you can see to the rear and to the side into the blind spot.

concave is shaped like the inside of a bowl. Convex is shaped like the outside of a bowl or dome.

The hint is to think "cave" inside a cave or bowl so conCAVE