China’s population is massive, almost 20% of the world’s total population. 71% of the Chinese population is between 15 and 65 years old, so the labour supply is still immense.
For context, in the graph below you can see other areas of the world squashed inside China.
For example, China is the world’s most populous country with 1.44 billion people. This is larger than a combination of South America, Europe (excluding Russia), the U.S. and Canada, and Australia and New Zealand.
P.S. don’t overlook India coming up fast behind China. Not for nothing do we call this “The Asian Century”.