How do green plants synthesise food?
Well, it's just like any other plant.
Green plants use a process called photosynthesis to make food from sunlight. Only green plants can make their own food. Other plants (like mushrooms) and animals do not make their own food, but get nutrition from green plants (or dead green plants). Animals do not make food from sunlight. If they could, they would not need to eat.
Green plants are autotrophs. They prepare their own food with the help of sunlight, carbon dioxide, water, minerals and the green pigment chlorophyll. Plants which doesn't have chlorophyll doesn't have green colour. So, they cannot make their own food. These plants and animals are called heterotrophs as they directly or indirectly depend on green plants for food.
The thing that helps green plants make food is chlorophyll, which is a green pigment that allows plants to absorb energy from the sun. The process in which plants make food is through photosynthesis and in this process plants use the energy from the sun to convert carbon dioxide into organic compounds.