Put more delicately, you are a great big multicellular eukaryote, each of your cells has (or had, in the case of red blood cells) an inner chamber called the nucleus, and you're full of mitochondria and other organelles. Your body is covered and filled with other organisms, many of them simple, tiny little single cell prokaryotes which make a living helping their gigantic, complicated host function. Like all the bacteria in your intestines that help you digest food. Their cells outnumber yours by a wide margin.
Bacteria technically live in the tube of "outside" on your inside. Digestive system is just one hole all the way through the body that your body interacts with just like the air in the lungs.
I read that comes don't actually eat grass. They have the extra stomachs, and the first stomach is basically a bacteria reactor that feeds on chewed up plant matter. As the bacteria reproduce they get sent to the next stomach which is what actually gives nutrients