In most of these designs the sun actually strikes the food. It is pleasing that the sun's energy is absorbed by the food we eat. Cookers or hot-plates reflect light to a focal point. Just the same as a magnifying glass. Use caution with a cooker, especially when children are around.
Intensified light can cause fires, burns and be harmful to the eyes.
Steamers work just like they sound. The cooker's heat boils water to make steam, which cooks the food. I have seen designs that are just a solar hot water collector. Simply directing steam to a box with a relief valve on it. Just like a pressure cooker, except the sun provides the power.