Oh, you wish to understand the process...... D'uh!! lol
Air is lifted up the lift tubes causing a flow of water. The displaced water comes from under the filter plate. That water comes from the tank, being drawn down through the gravel and bio-bed removing particles and ammonia. It's just a big circle. That is why, when doing water changes, it is best to sweep the gravel from time to time. That keeps the substrate clear and prevents it from compacting.