In the UK, their equivalent of high school kids take their AS and A level exams in very focused subjects usually 3 - 4 subjects. So a student interested in studying a STEM major in university will take Biology, Chemistry, Physics and Math or Chemistry, Physic, Math and Further Math. They will also be expected to have pursued extra-curricular activities related to the major they have applied for for instance a future medical student should have volunteered in hospitals or similar healthcare settings. Prior to high school, UK students study a lot more than those subjects through and then sit exams (the GCSE exams) on them. This helps them to determine which subjects they like and are likely to do well in.
GCSEs are typically written around age 16; A level is written around age 18. My experience many years ago was that A level STEM subjects essentially covered the first year and a half of the corresponding university courses in North America.
