Not much of a surprise considering China is likely to remain the largest commercial ship builder in the world and probably also the largest navy vessel builder in the next 2 decades. Current estimates for China's planned naval vessels alone are staggering: 60-80 new 10,000-ton class destroyers, 8-10 90,000-ton+ class aircraft carriers with electro-magnetic launching systems (flat tops), 10+ strategic nuclear subs (boomers), 14+ nuclear attack subs, 20+ conventional subs, 10-20 amphibious landing ships, 20+ 4k/6k ton class frigates, ... China's high speed rail system will more than double in size. China's automobile output will continue to widen its lead over the rest of the world. China is still building skyscrapers, express ways, bridges etc en masse. Those industrial sectors all consume large quantities of steel.
They borrowed Japan's 1980's memory chip playbook: In a recession/slowdown, US and the rest of the world cutback whereas Japan chip makers invested in building new capacities so when demands recovered, they gained market share.
The way I see, the 19 days exclude - foundation work which could take years depending on ground condition - mechanical, electrical, plumbing works (some of which might be already in the module) - finishing touch to the facade, tiles for interior surfaces - interior decoration - lift, escalator, sprinkler system, pumps, cooling/heating system ... installation ... the 19 days include - only stacking of floor/ceiling/wall/window modules which were manufactured offsite
Carbon emissions for steel production in China is twice what it is in the United States for the same quantity of steel. While Chinese steel is used for plenty of price sensitive commodity consumer items - there are a myriad of critical engineering spec applications where German, US, Japanese, and Korean steel alloys are preferred.