While some of the money foreign workers make is sent back home to family in another country, a lot of it is spent right here in the US, boosting our economy. Often, those foreign workers end up moving here permanently, meaning more of their earned money is spent in this country.

A vast majority of these foreign workers become huge net positives for the US economy. The economy doesn't work like you seem to think it does; there aren't some set number of jobs that need to be divided up amongst all the people. These imported workers take a job, yes, but they also a new consumer of goods and services.