去年（きょねん）= "last year", as in: the year before the current year.

先年（せんねん）= "[a few] years ago": some years prior to this one.

前年（ぜんねん）= "(the) year before" / "(the) previous year", relative to a particular occassion.

Context, usually. If the conversation were about the speaker, this presupposes the speaker has amnesia. Because most questions here assume the topic is about the listener, it's safer to presuppose you're getting to know the other person; in this case, you're asking the listener what they did last year.

Unlike English, which tends to be specific and requires an explicit subject, Japanese tends to flow with the context. If you see a question here, it's better to assume you're asking for information about the listener from the listener (unless it begins with "watashi wa" or "boku wa," or if the conversation is about the speaker's amnesia, which is a rare topic anyway).

I think it's because questions are usually asked to someone else.

しました means "did", not "happened". The difference in meaning is important.