Question

What provides the force on the person in the passenger seat?

Answer 1

The forces that make a passenger speed up, slow down, or
turn a curve are the same forces that have the same effect
on the driver and anybody else in the car.

-- Speeding up . . .

              the back of the seat
              friction between the car seat and the seat of your pants

-- Slowing down . . .

              the seat belt
              friction between the car seat and the seat of your pants

-- Turning away from a straight line . . .
   
              the seat belt
              friction between the car seat and the seat of your pants
              the door, or whatever or whomever you're leaning against