It grows almost everywhere inside your tank. But the biological media inside your filter is where a good majority of it will grow.
The bio media is designed to provide a large surface area for bacteria to grow, while your filter provides a constant supply of well oxygenated water.
The sponge in your filter is used to keep the water physically clean floating debris so that the water is visually clean
Additionally, the carbon in your filter helps remove coloration smells and other toxins that may be present at the bacteria simply won’t touch effectively creating a trifecta of aquarium