In some cases, more energy is burned to stay warm. In some cases extra illusulation is added to stay warm. In some cases cells go into inactive states. In some cases cells produce intracellular antifreeze. Some cells cease metabolizing.
Some cells continue to metabolize but at slower rates.
Most warm blooded animals have multiple strategies at different levels in their structures and their physiologies. You will learn much more about these strategies later in you studies of physiology. Overall, there are probably as many strategies as there are types of organisms. Please keep asking them. In some cases, more energy is burned to stay warm.