Urgent! Why do domestic immigrants lead to a large population? It must be solved today!

I think your question is very representative. It's a bit like the topic of human geography in high school ... I think it should be that the population of those economically underdeveloped provinces moved to those economically developed provinces, and the immigrants from economically developed provinces developed, and the material living standards also improved, which also led to the change of fertility concept. Immigrants in their hometown may find it difficult to have multiple children because of economic problems. When I arrived in a rich province, my living welfare was improved, and I naturally had the idea of having a second child. Therefore, domestic migration has caused the spatial imbalance of fertility rate, that is, the fertility rate in developed areas is high. Finally, the population will increase.

Personal opinion is not necessarily the best explanation ~ ~