I'm majoring in physics (for example ) means "Моя специализация - физика". It is a degree you get in college/ university.

Your major (the thing you are majoring in) is the main subject you study at college/university. In the UK, we don't have majors/minors, as far as I'm aware. We get accepted at a university to study a specific course, although this can change once started to a similar one and it can include unrelated modules, such as a foreign language.