Do even expensive restaurants hire immigrant helpers? id so, is there greater risk of food poisoning?

No: Immigrants come from many countries and you can't generalize food preparation hygeine on country of origin, it depends on the training and supervision of the workers, american or not.

Back office employee: Most restaurants including expensive ones hire minority and immigrant employees as they generally are good workers and will work for less money. Food poisoning is a constant worry in any restaurant and most have strict internal rules to prevent this. These rules are more likely to be followed in an expensive restaurant so enjoy your experience there.