“China is screwed, because their population may be as low as 800 million by those who have knowledge.”
That’s more than twice the US, so hard to see how they’re ‘screwed’.
If China has been borrowing money based on a population of 1.4 billion, but there are only 800 million, there isn’t enough people to pay back that debt.
Also, it’s not just population, but the demographics of the population. Old people are not producers to fund the government, but consumers. If there are too many old people, there are not enough people to support them and the young, who are supporting them, will not have enough to support themselves when they reach retirement age.