China has an enormous and rapidly growing AI space. I would argue growing much, much faster than the US space where in a lot of cases the hustle has replaced the engineering (see: OpenAI).
No, they aren't all working in the USA, and in many cases those that were are leaving the US and moving back to China.
You only see the ones working in the USA and not the ones working in China. The USA still pays better than China, and has more stomach for immigration (for now…Trump is working on making that worse), but when I was working in Beijing a lot of engineers from Japan, Korea, Taiwan, were there specifically for the higher pay.
No, they aren't all working in the USA, and in many cases those that were are leaving the US and moving back to China.