First, Yongjia crossed the south, and the great integration of North and South nationalities brought advanced production technology and a large number of laborers.
Second, the Anshi rebellion, a large number of northern farmers moved south to avoid disasters, objectively promoted the development of the south;
Third, the establishment of the partial security regime in the Southern Song Dynasty was also a large-scale immigration activity.
As for modern immigration or migration, there are many reasons because as long as there is war, there will be population migration, which is inevitable. Narrative like "Dongdu" is just an episode of people's migration.