I don’t use these services, but when I see news stories about them they are usually foreigners - and I suspect they are supplementing regular jobs with this. Users I know swear by the program, but so would the rickshaw rider a century ago.
Around the country, using these whenever I travel, rarely have I encountered obvious foreign or immigrant workers. Most are clearly lifelong US drivers, mix of mid- and sometimes older drivers. Most notable observation is that most are new to driving for Uber.
I use Uber or Lyft quite often (but not in NYC). I'd say that about 20% are foreigners/immigrants... certainly larger than the population at large, but there's no shortage of Americans (mostly college kids or older folks) who are doing the driving. It's not a terrible way to make extra disposable cash. Sit in your own car as much or as little as you want, only take the trips that you want, only work when you want, no boss to answer to, and make a decent "hourly wage" overall... listen to your radio, or enjoy light polite conversation with strangers for 10-20 min at a time... get paid faster than any "normal" job...
Most drivers enjoy the opportunity very much.