Two ways. The primary way is called "crippling student debt", which they pay for... probably the remainder of the lives, honestly.
The second way is not going to the buggeroff expensive colleges. It's in-state (so much better price), but tuition/books for where I'm at right now has been like maybe 5-6k for a year. Would be more a bit more expensive for higher level courses and that, of course, isn't including cost of living stuff, but... if you're paying 50k a year, you're probably going to one of those pointlessly high end ones you don't actually need to go to to get a degree and/or work.
The remaining ways, at least for the buggeroff expensive colleges, is scholarships (somewhat) and rich parents, more or less, unless you're somehow already fairly well off (and in that case, why the bloody hell are you going to a 50k/year school? You should know better by now.).
E: Though it's been a while since I was aware of just what the cost for out-of-state/non-US-citizen tuition is, beyond "usually hellishly more expensive". Colleges tend to fleece out of staters like goddamn. Even worse than they do in-state folks.
E2: Ooh, checked. Yeah, I'm at a fairly affordable one and non-resident tuition ranges from 3.7 to 5 point somethingthree times that of resident tuition. Think there's exchange program stuff you can occasionally work through to reduce costs, but... yeah, if you're coming from somewhere else it gets hellishly expensive. Finding a way to avoid that is... usually a good idea. If it's possible.