China has almost no immigrants because it’s for all intents and purposes almost impossible to immigrate to China.
I am one of very few foreigners in history who has held a ‘Chinese’ passport and therefore Chinese. I’m not Chinese now though . Long story .
Some good comments above, modern China wasn’t predicted when the 1 child policy was introduced. The 1 child policy was very successful in helping China catch up with the needs of its population though.
Women now have so many more options. The trend of marrying later also affects the fertility rate greatly.
Look at the Philippines at the opposite end, they simply cannot keep with the population growth. Economic growth is not translating across to much of the population with vast numbers forced to emigrate.
Falling birth rates is common across much of Asia, Vietnam and Thailand included. Generally its a very good thing