How do China high school graduates who immigrated to the United States apply for American universities?

Take the sat exam. China students require TOEFL, but since they have been naturalized in the United States, they don't need TOEFL. The SAT is a bit difficult, especially the reading part. It is much easier to apply for a scholarship after entering the country than for international students.

See what kind of university you want to go to, and then go to official website to find out what materials you need to apply for (good university materials are generally different, composition topics and so on).