Earth pulls all objects downward by the same gravitational acceleration of 9.8 m/s². But the force of gravity exerted by Earth on an object still depends on its mass. An object with a greater mass has a greater force of gravity (also called weight).
Flat-Earthers claim that if greater mass means a greater force of gravity, a bowling ball in a vacuum should fall faster than a feather. In reality, they confuse acceleration with force. Both fall at the same speed because the gravitational acceleration is the same on both, but their forces of gravity are different.
The Earth is also affected by the mass of both objects. The bowling ball exerts the same magnitude of gravitational force on Earth as the Earth to the bowling ball but in the opposite direction. We do not see the Earth accelerating upwards because the Earth is considerably more massive than the bowling ball. While the Earth accelerates downward by 9.8 m/s², the bowling ball can only accelerate Earth upward by 1.6e-23 m/s², practically meaningless. The same thing also happens between the feather and the Earth.
References
- Newton’s law of universal gravitation – Wikipedia
- Gravitational acceleration – Wikipedia