When the car is stationary, the passengers are also stationary. They have inertia, meaning they resist being put into motion. When the car accelerates, the car exerts a force on the passengers (through their seats and seatbelts), overcoming their inertia and causing them to accelerate along with the car.
Once the car is moving at a constant speed, the passengers are also moving at that same constant speed. They still possess inertia, resisting any change to that constant velocity. They only experience forces if the car's velocity changes (accelerates, decelerates, or turns). In the absence of external forces (like friction or the car braking), they would continue moving at the same speed and direction indefinitely, as per Newton's first law of motion (the law of inertia).
