The DOL has salary brackets based on job title, experience, and location but I'm sure there's a way for employers to stick H1B workers into experience brackets below where they should be, locate their H1B workers in offices in cheaper locations, or just simply pay them less than their domestic coworkers while still meeting the salary requirements.

I don't know if discriminating against visa holders is illegal, but most visa holders are minorities so this would be a violation of the civil rights act

What I’ve seen is they get hired at a junior level. Then the promotion opportunities are few and far between.