What determines natural population growth?

It depends on a combination of fertility and mortality. If the first indicator is higher than the second, then a natural increase is obtained. The easiest way to achieve it is all poor and agrarian countries like Afghanistan, Niger and Somalia (or, for example, Russia at the beginning of the 20th century). It is more difficult with industrial and urbanized societies, in them the birth rate is falling and the death rate is increasing due to the aging of the population, the example is Japan, which has come to a negative natural increase.