I was a hiring manager. H1B candidates got exactly the same wage as native born. There were no differences.
It's common for the prevailing wage to be double what that person could earn in their country of origin.
* The expected marginal increase in supply, from immigrants looking for jobs in F
* The expected marginal increase in demand, from immigrants (or non-immigrants identifying an opportunity) starting companies that need experts in F
And how strongly does that delta correlate with average salary in F?
You don't need the carrot of a green card or the stick of deportation to make a software engineer on an H1B accept a depressed wage, because it's not depressed relative to their alternatives.