It's pretty obvious for Hell's Kitchen they put in actors who act like dumbasses (Raj) and get yelled at for comic relief
Then there's the real people who are actually trying to cook. You can tell by seeing how Ramsey talks to the person if they make a mistake
No they are all chefs, they all have certifiable credentials and everything that you can find pretty easily.
They are all mostly just, short order cooks, line cooks, prep cooks, personal cooks, etc etc.
Almost none of them have experience in fine dining, which is why you see them fuck up time and time again. At most they serve like, 20 seater places. Or do catering. Where they can afford longer wait times and to fuck up occasionally.
He's so harsh on all of them because they are, ostensibly, professionals and he believes firmly that if you call yourself a professional, you should be able to handle the additional load and stress.
A lot of them just break down or don't have the actual talent to carry themselves.