Some turtles live where it is warm all the time. They stay active all winter long. But turtles who live where it gets cold and snowy have a way to keep themselves alive. They can hibernate . Different kinds of turtles hibernate in different ways.
Most water turtles go deep into the pond and snuggle down into some mud and leaves at the bottom.
Then they let themselves get cold. Their bodies slow down so they don't need to eat anymore. Their hearts slow down too so that they beat only once every few minutes. They stop breathing through their lungs. Because their bodies are running at such a slow speed, they don't need much oxygen , but they do need some.