For Oxford and Cambridge, you apply directly to the college. But the colleges are very, very small. They may only have one or two spaces for each course, each year. Now, if you're an excellent candidate, but there are more excellent candidates at that particular college, then you may get sent to a different college. If a college thinks you are an excellent student, and you've only just missed out on a place there, because of the large number of excellent students applying this year, but on any other year they would have accepted you, because you are excellent, then at the Cambridge they'll put you into a pool or at Oxford it's called reallocation.
Colleges that have spaces, or feel that the people that applied directly to them weren't quite up to standard, can look at pooled or reallocated students. Because what each college is looking for are the best students for them. Not ones have that have necessarily applied directly, not ones that necessarily chose that college. They would rather have a better student who applied to a different college, as opposed to somebody who applied directly to them but didn't quite meet up to their standards.
Oxford and Cambridge do their initial interviews in the first few weeks of December. If a college has a large number of really good applicants that year or doesn't think you are quite the right fit for a college but you would fit in really well somewhere else, then they can choose to send you to a different college. Oxford and Cambridge do this ever so slightly differently.
Oxford do everything in one day. If you go for an interview at Oxford, you might have an interview at the college you applied to and then you might have an interview at a different college, as well. All of the Oxford interviews, the initial interviews and then the reallocation interviews are all done in one day.
Cambridge do things across two days. They have the initial interviews in December then applicants go into a pool, and then in the beginning of January, students might be get called back for a second interview by a second college. Cambridge send about a quarter of its applicants to the pool and then about a quarter of those end up getting a place at a different college.
If you get sent to a pool, or if at Oxford you get sent to a different college to be interviewed for reallocation, you may get an offer from your initial college, you may get an offer from a new college, or you may get rejected. And there is absolutely nothing you can do about this. You don't get a second choice of college. You have no influence over which college gives you an offer. You can state your preference when you apply, but it is just a preference.