A small example of pointers of pointers in comparison pointers of instances.

pointer.c

struct Wheel {
  int number;
};

struct Car {
  struct Wheel* wheel_pointer;
  struct Wheel wheel_instance;
};

int main() {
  struct Car car;
  struct Wheel wheel;
  struct Car* c_pointer = &car;
  car.wheel_pointer = &wheel;

  c_pointer->wheel_pointer->number = 4;
  c_pointer->wheel_instance.number = 2;
}