World Population Prospects - Population Division - United Nations
2/ When most people look at that graph they overweight the importance of the top line that shows unending growth. The top line is actually beyond the 95th percentile guess. The median has peak population in ~2100 These predictions were from 2017 Source
What are the factors which have led to such a population projection for Bulgaria?
Nigeria, a country a third larger than Texas, will (likely) have a larger population than the United States 30 years from now.