How do living organisms use inhaled oxygen?

Not all living organisms need oxygen. Most animals depend on this gas. But viruses do not need oxygen. The same goes for some types of mushrooms. Plants absorb carbon dioxide. But why then do animals need oxygen? The point is that oxygen provides the vital activity of cells. Without it, oxygen starvation of the body begins and the process of cell death begins very quickly. The most dangerous thing is when blood does not enter the brain, since it is the brain that regulates all life processes and needs a large amount of oxygen.