American scientists are equivalent to assistant researchers in China. As far as I know, research scientists can take the place of professors and help them manage and guide graduate students, instead of undertaking courses alone. I don't know who pays the salaries of research scientists. Professors seem to have a lot of autonomy, and they can directly recruit scientific research scientists without a school interview. It is possible that the professor has a lot of research funds and is responsible for paying research scientists.

Of course, this does not mean that the level of research scientists is not good. There is a doctoral supervisor in China who has been a research scientist in the United States for a long time. Because there is only one professor in each direction, the level of many research scientists is much better than that of domestic professors, but the level of some research scientists is similar to that of postdoctoral students who have just left the station.

Perhaps American universities are similar to Germany in the field of geosciences, with only one professor and some only one associate professor in each research direction. In the CEMI research group of the University of Utah, all the staff except one professor are research scientists. There used to be an associate professor in this group, which was also very good. Before becoming a full professor, he went to other departments and changed his research direction. In another university, the research group is very small, so there is only one associate professor. There is a full professor and an associate professor in a research institute at the University of California, San Diego, and the rest are research scientists.