A " birthrate has declined by 20%" do not mean there is not an increase in population, if it was high enough to begin with you can still have an increase it is just slower.

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Even if you have a birth rate that over a long time would result in a stable population it can still increase. Let say it would end with 10 000 births and 10 000 deaths per year. The population is 10 000 x average lifespan, so if the life expectancy increases the population can grow even if the birth rate is constant.

The average human lifespan on earth during the 20th century increased from 48 to 74 years for men, and from 51 to almost 80 years for women. that is close to 35% increase for both. That is a major part of the population increase on the earth

But there is another major factor for population density other than birth rate is migration both inside and between countries.

So even if the birth rate in your country would result in minimal growth migration from other countries can increase the population.

The general trend today is people moving to large cities from the countryside and smaller cities. So if you live in a large city you will see a population increase that primarily is a result of people moving there. The change is not primarily because if birth but migration.

If you were out in the countryside you would likely see a population decrease. It is a result of people moving to cities. Because young people are more likely to move than old the demographics can become quite uneven and as a result, the number of deaths can be a lot higher than the number of births.