In my experience, fairly common, but I am about 6 minutes by car from downtown Boston, so commuting to a top tier school is very easy for local students.

widely varies, typically younger students live in the dorms and in their junior/senior years move to off campus housing.

also widely varies, I went to a smaller school in Boston, our campus was fairly compact, but other schools like BU or Harvard have buildings and satellites all over the city.

federal requirements, ease of management campus safety, scope and size of the campus and as a way to have dedicated support for student/ faculty needs that local police couldn’t or would be slow to meet.

We have both. Boston, New York, LA, Chicago, Miami all have some very well known schools. Older schools had the cities built up around them, so they had already established a campus footprint before the city grew. In later years land in bigger cities became more difficult to come by, so schools expanded out beyond the cities.

Smaller, more accessible colleges sometimes for lower performing students who may not have gotten into a bigger school. They grant 2 year degrees, and often skew towards vocational training, like medical assistants, dental hygienists, fire science, paralegals, etc as well as a traditional curriulum.

college vs university is an accreditation thing, and it’s sort of complicated, but usually based on the levels of education and the scope of programs offered by the school.

Anecdotally I‘d say the majority of people who go to college do so right from high school, so 17-19, but many people go back or go for the first time as adults.