A simple way to remember is: egg white is almost pure protein and water, while the yolk is a rich mix of fats (lipids) and protein. The fats are the most significant component of the yolk by weight.
The egg yolk's primary function is to provide nutrients for the developing embryo. By dry weight, it is composed mainly of lipids (fats), which make up over 60% of the yolk's dry matter and serve as the main energy source. While the yolk is also a rich source of proteins (about 33% of dry matter) and vitamins, lipids are the most abundant component.