Older style toilets – water fills the bowl until the level is higher than that of the drain. Once that happens, the water sucks all your “stuff” down the pipe just like siphoning works. (Put a pot of water up high and stick a tube in it with the other end lower than the pot, suck on the tube until water flows and watch the pot drain completely.)
Newer toilets – there’s a “jet” that kickstarts the above process by forcing water directly into the drain rather than waiting for the bowl to fill.
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