the migration during the last decades from rural China into the cities has been the largest population migration in the history (something like 800M people moved). One can imagine how future generations will be learning about great migrations - like that of the Goths and Huns in the 4th century or this migration in China in the 20-21st century and the resulting large scale effects on the history lasting for centuries.

Btw, interesting depiction of Shanghai - exterritorial status of the foreign concessions for example - in 1937 in the movie https://en.wikipedia.org/wiki/The_Eight_Hundred . In the West we sometimes miss that the 2nd Sino-Japanese war can be naturally considered part of the WWII which thus de-facto was already going in the 1937.