Chinese people (the ordinary ones, not the rich) have almost nowhere to invest their money somewhat secure because of capital control and hard control of the stock market by the CCP.

Coupled with a culturally ingrained appreciation for real estate, this led to a huge housing boom - it's the only viable investment and there's the expectation that young men own real estate. And this turned into a huge bubble where shoddy, unusable buildings are built in hastily "developed" areas that will never house people.

This bubble was about to burst, but since that could endanger the CCP, they managed to turn that into a rapid deflation instead.

Thats the intention, but a lot of buildings are getting half-finished, then sold when they lose funding... just to be demolished so they can start all over again.

No, frequently chinese devlopments are built far in advance of the population moving there. There are multiple fully built modern ghost cities. That may not be the case here in Sanya City, though. I haven't looked into it.

Yes and no. There are a lot of places in China that are high population density but also hundreds of completely empty skyscrapers.

Kinda like how in the US you have entire neighborhoods completely empty built for the sole purpose of artificially raising house prices in the area.