Not American either, but in the US many schools use/used standardized admission tests (SAT/ACT) on top of things like HS GPA/grades.
There are many countries, especially in Europe, where entrance/admission tests are not a thing.
Yeah, in England only certain universities like Oxford and certain subjects like Mathematics have separate entrance exams.
That said, the Sixth Form exams are mostly standardised with only a few different exam boards for the entire country, so the Sixth Form grades end up being something akin to standardised tests anyway.