It will differ in different places, but the big impact on population growth was the availability of adequate food. The growing of staples such as rice, wheat, potatoes finally created a surplus of calories so people could start to spend time on other endeavors besides subsistence farming. Carpentry, ironwork, trading, etc. all can come about if there is enough food. For example, the population in Scotland and Ireland exploded after potatoes were brought back from the Americas. Potatoes could grow in the often poor soils, and provided adequate calories. When the potato famine came, millions died because there just was not enough food. (I am leaving out the political implications here as they are not germane to the topic.)
They discovered commerce. You no longer had to do it all - raise a crop, animals, shelter, etc. You could do what you did best - construction, butcher, farmer, etc. - and trade with other people who had commodities you didn't. If you could bring together everything you needed to survive, you could stay in one place and grow.