It is probably more precise to just stick with the term "US-legal" instead of thinking about birth. That's a better term I should have used.

The mechanism is if there's one perfect candidate of the right age, race and caste but the person isn't US-legal, then companies will be forced to consider people of the wrong age, race or caste. Maybe not right away, but when the supply of the perfect age/race/caste US-legal people run out.

In my experience, the visas usually accentuate the distorted racial makeup of tech companies because they all go to overrepresented countries.