Moving back to my home country (China) after my OPT. I am able to keep working for the same company (US company) while I am in China. How does the tax work in this case?

I think I have answered your other post. However, the  new wrinkle is your working for  a US entity while resident in China.  For this part , your earnings in China do not come US taxes because the work is being performed by a resident of another country  and in that country ( even if you are remote worker).