The broth is made by simmering kombu seaweed in water. There are many variations. Some will add soup stock to the seaweed broth; some will use a stock based on fish.
A firmer style of tofu such as Yaki-dofu is used in medium-sized pieces. The tofu isn’t really cooked, just simmered in the broth for 2 to 3 minutes until it is warm in the middle, at which point it will rise to the surface.
In restaurants, it will be served in the dish of broth. You capture the chunks of tofu with your chopsticks and dip into them a dipping sauce on the side before eating. It is usually served with some garnishes as well.
Yudofu is very similar to the dish called “Hiya Yakko”, but for Hiya Yakko the tofu is served cold and uncooked.