I had a friend who did this for a living, he had two phones so he would run Uber Eats and Doordash at the same time in order to try and optimize, and even then he spent a lot of time just sitting around because these apps are extremely inefficient (and later they started cracking down on people trying to increase their pay by doing both, dunno if it’s still doable now).
According to this document from 2021, most of them are “assistants” and “aides” of various types, although it seems there are a few thousand registered nurses and other licensed healthcare providers and technicians. Maybe they get licensed on a visa and then overstay the visa?