Why is it impossible to determine whether a body is moving or not without pointing to other bodies?

Why is it impossible to determine whether a body is moving or not without pointing to other bodies? Does the person standing on the sidewalk move relative to the passing bus? what can be said about the movement of the car relative to the combine, from the bunker of which the grain is reloaded into the car body without stopping the combine?

To judge the movement of a body, it is necessary to know whether the position of a given body changes among the bodies around it.
A person standing on the sidewalk, relative to the passing
the bus does not move mechanically, i.e. is at rest, and the bus is moving relative to the human body;
if the position of the car relative to the harvester, from the bunker of which the grain is reloaded into the car body without stopping the harvester, changes, then it moves relative to these bodies, if the harvester moves and the car moves, then they move relative to each other.