People move relative to the earth (reference object) when running on the ground and relative to the conveyor belt (reference object) on the treadmill when running on the treadmill. Because the conveyor belt moves relative to the earth, people are still relative to the ground.

The conveyor belt on the treadmill keeps turning, making people unable to run. If the imaginary conveyor belt is infinitely long and does not rotate, people will run from one point of the conveyor belt to another; Now, people don't move (here, people as particles don't move, not including the moving internal system of the conveyor belt, and people also run from one point on the infinite conveyor belt to another. Both results are the same. In reality, the conveyor belt is not infinite, but rotates circularly, which is equivalent to the conclusion of analyzing the infinite conveyor belt.