Lots of developed countries are facing population decline as birth rates continue to decrease, but China’s One Child policy really kicked things off for them early, relative to their level of development.
And to make matters worse, they don’t have immigration flow to stave off the decline. Economically, population decline is a serious problem. You have cities getting abandoned as housing is no longer needed and being maintained, and you have not enough working-age people trying to take care of too many old people.