They just have so many people. They could replace entire major foreign industries and still have people left over.
Couple that with a government willing to do whatever infrastructure and monitary policy and you end up with a ton of growth.
The US may have just had a corporate takeover and will end probably end up looking more like China or Russia going forward.
There are a lot of natural resources here and if you're not particularly concerned with the local environment or economy, you could probably get a lot done.
I also can't imagine a future without China having the largest consumer class going forward and that probably represents a valuable market.
https://youtu.be/N2lpoRaUR8E