Absolutely, and a useful clarification.
They definitely don’t. As you point out, there can be different subject requirements, recommendations, or just observed practices. Sometimes there are special admissions exams. Obviously at some universities there are interviews (which are real, substantive interviews conducted by the tutors in the course). And so on.
So I definitely do not think St Andrews is actually a harder admit than Oxbridge whenever the Guardian table shows a course having a higher entry tariff.
But I do think it is fair to conclude that such a St Andrews course is at least high up overall in terms of selectivity for UK applicants, even more so Scottish.