How can the plants under ocean bed make food to live without sunlight
Answers
Deep sea plants do not photosynthesis as there is lack of light. They make their own food by a process called chemo-synthesis: it is the process of oxidization of high-energy inorganic compounds (hydrogen gas, ammonia, nitrates, and sulfides). this is only practiced by certain microbes underwater where sunlight is not readily available. however the maximum depth common aquatic plants can grow is 12m this is because beyond this limit the fluid pressure is unbearably for aquatic plant-life.
Deep sea creatures are not capable of photosynthesis since, as you mentioned, they do not have access to sunlight. Instead, some of them create their own food through chemosynthesis: oxidization of high-energy inorganic compounds (hydrogen gas, ammonia, nitrates, and sulfides). Those chemosynthetic organisms are eaten by predators which are themselves eaten by other predators and so on and so forth.