What causes hot air balloons to fly? It has weight and pushes down on things on Earth and it decrease as altitude increases.

The hot air balloons fly in the air due to buoyancy. There are two forces which tend to act on the hot air balloon. The gravitational force which tends to act downwards. The air pressure force which tends to act upwards. The air caught inside the balloon is heated up by a burner due to which it becomes less dense than the surrounding air and the balloon moves in the upward direction.