Those working for cash under the table wire money back HOME.
They are “low income” because they choose to do without the cash they are taking in. They are banking it. Billions are wired to Mexico annually.
The ‘Mexico’ label was fair to use in the 1990s, but I think in the past twenty years....a large segment of the incoming crowd are from Honduras, Guatemala, El Salvador, and Belize.
You can also note in recent weeks that African migrants have been noted crossing the border. Rumors suggest that freighters are carrying across the Atlantic for a price, and a smuggler route is being developed. Wouldn’t surprise me in a year to see 3,000 Africans a month crossing the border using this route.