The first is that all of their buildings are designed by machines, and they're just putting things together. That seems unlikely though, because it doesn't explain how they're building gazebos.
The second theory is that they stole the designs from some other country, who stole the designs from another country, etc., until we get back to America at the very beginning of this process. This is also unlikely though because I don't think we've ever been able to build anything as amazing as what China is building right now!
So my third theory is that they are using super advanced CAD software that allows them to design things like gazebos without having any skill or training in CAD design whatsoever. This would make sense if China had been developing their own CAD system since they were founded in 1949 (or whatever), but again this seems highly unlikely because it doesn't explain why our own country's CAD software isn't capable of designing things like this (or any other similar projects).