word choice - "In college" versus "at college" versus "at university"

Which one is more correct: “works at a university” or “works in a university”?

It seems that only in the U.S. one says that they are or were "in college", even though the person attended a university, such as the University of Iowa. You are either in college, or majored in such and such while in college. You hear the phrase "at college" less often, but never "at university" in the U.S.
Elsewhere, even next door in Canada, the equivalent phrase seems to be "at university": "I majored in English while at university." I don't think I have ever heard or read the phrase "in university" used.
Likewise, in the U.S., one says, for example, "I will be going to college next year" whereas elsewhere one seems to say "I will be going to university next year."

Is there anywhere else in the world besides the U.S. where the phrase "in college" is used instead of "at university"? How did the American English usage come about?

Answer

There was nothing called a "university" for the first century and a half of English settlement in North America, and for even longer many of the best-known such institutions were known as "colleges" (some, like Dartmouth, remain so to this day). It should be no surprise then that college, not university, became the generic term for post-secondary education.

Universities like Cambridge and Oxford— made up of various autonomous colleges— had already stood for centuries when the Massachusetts Bay Colony established its "New College" in the 1630s. This tiny institution would not have been recognizable as a university; it was, at best, like one of their constituent colleges. People like John Harvard, a graduate of Emmanuel College, Cambridge, would have understood this distinction. His endowment turned New College into Harvard College, not somehow suddenly Harvard University.

Even at the Declaration of Independence, none of the institutions of higher learning in the United States were chartered as universities. Where they were reorganized as such, undergraduates usually remained under a single faculty, so a "college education" was synonymous with a "university education"— Yale College, Princeton College, and so on persist to this day. Some universities retained "College" in the overall institution's name, e.g. the College of William & Mary.

All these factors would have further entrenched "college" as the generic term for that phase of education, regardless of whether it is undertaken at a university, college, institution, academy, conservatory, polytechnic institute, and so on. One is in college (i.e. enrolled as a postsecondary student) or at college (i.e. away from home on account of enrollment in some distant postsecondary program) just as one would be in elementary school, in apprenticeship, or in seminary.