https://ourworldindata.org/demographic-transition
Population growth is determined by births and deaths. Every country has seen very substantial changes in both: mortality and fertility rates have fallen across the world.
But declining mortality rates and declining fertility rates alone do not explain why populations grow. If these changes happened at the same time, the size of the population would not increase. What is crucial is the timing at which mortality and fertility changes.
The model that explains why countries go through a period of rapid population growth is called the ‘demographic transition’. It is shown in the schematic figure. It is a beautifully simple model that describes the observed pattern in countries around the world and is one of the great insights of demography.