one is that Chinese "city" boundaries also generally include a lot of rural land, mountains, etc. Chongqing is the size of Austria.

The other is that while the buildings are certainly tall, China also does a lot of "tower-in-the-park" style development where the plazas and landscaping in between tall buildings decrease overall density.

PRC cities are not particularly dense. Population densities in cities within a country tend to follow zipfs law, which would predict BJ and SH to be 3-4x larger than it currently is. Many economists and urban planners was suggesting PRC should densify tier1 about 10 years ago. But hukou anbd industrial policy seems to be designed to limit megacity sizes to redirect popuation towards growing 3rd, 4th+ tier cities into their own economic hubs. IMO trying to avoid SKR/JP where everyone rushes to a few economically viable regions.

China is deurbanizing and dedensifying. It's a popular myth that Chinese cities are dense. Density peaked a decade ago.

This will spell economic worries someway down the road as maintenance upkeep costs start to kick in.

What is your experience ? China cities are noticeably less dense than other cities in Asia (Manilla, Delhi..) or even Paris

Most of Manila is an endless sprawl low-rise housing and single-story slums, with a few areas of high-rise buildings (Makati, BGC, etc). In the average large Chinese city, more or less all the housing is high-rise.

how much of that housing is occupied vs investment, and how large are the apartments? slums get quite packed.