Basically from their parents and their parent's friends. It's kind of funny because I can see the OP's point: on the one hand people are sold this idea of a meritocracy. If you think about "the American dream", anyone no matter what their background can "make it" through hard work. But there is an unspoken implication there: that you can "make it". What happens after you "make it"? Presumably you're on easy street. You don't have to work hard any more. Your family is taken care of. They don't have to work hard any more.
I tend to think of the "American dream" more like a class lottery. Sort of like the movie Ratatouille, it's not that anyone can be in the upper class, but people from the upper class may have come from anywhere. I think of people like basketball players who come out of poverty. Their mother used to clean toilets 14 hours a day and now that her son is a famous basketball player she can spend her days relaxing in a big house and she never has to clean toilets again.
But the OP is correct. No matter how people made it to the upper class, once they are there their family has an advantage. Even if you start with a totally fair meritocracy where people get their just rewards, because they want to provide for their children (and their children's children) you end up with a class of "old money" who own and control most things of importance.
I'm not sure there is a way around it other than massive inheritance taxes and high inflation, which are historically not super popular ;-)
By that description, we can draw a line around the people born in regular old middle class households, and another line around the wealthy, and imagine them trading with one another, much like poor countries in the mid to late 20th century, that are now wealthy. This should demonstrate there is nothing holding back the rest of this country's population from getting wealthy too, while some rich kids inherit from their parents, or go to some rich-kids-only-(except-not-exactly) university.
A different situation is if some wealthy people engineer things so as to effect transfer payments from the middle/poor classes (or just everybody in general) to themselves. I.e., rent-seeking, which obviously exists. While I don't like legacy admissions, afaict it's not rent-seeking.
