How do demographers model world population? Is this different from how they model, say, the population of the United States?

Population Growth Model

How do demographers model world population? Is this different from how they model, say, the population of the United States? How does this process compare with that used by biologists in determining the size of a future salmon spawn? Compare and contrast the process demographers use to model human population growth with that which biologists use to model animal populations.
Real life example, connection and conclusion?