When you run both your washer and dryer at the same time, the water in the washer remains in the washer and goes through its regular washing and draining cycles. The water is used to clean the clothes, and then it is drained out through the machine's plumbing system. The washer will fill with clean water for rinsing and draining until the washing cycle is complete.
Meanwhile, the dryer operates separately and does not directly interact with the water from the washer. Its purpose is to remove moisture from the clothes after they have been washed. The dryer uses heated air to evaporate the water from the wet clothes, turning it into water vapor, and then expels the moist air through a vent or lint trap. The water vapor is then typically vented outside of your home.
So, when running both the washer and dryer at once, the water from the washer will be drained through its usual process, while the dryer will be eliminating moisture from the clothes by venting the water vapor outside. They are separate systems that work independently to accomplish their respective tasks.