"...as far as how getting into in/college works"
As I understand things in the US (and please correct me if I'm wrong), college entry is largely based on SAT scores and then GPA and/or AP scores.
In the UK, entry is based solely on the British equivalent of AP scores (although there are some specialist tests for entry into, for example, medicine and law eg the UK has the UCAT for medicine which is the equivalent of the MCAT in the USA).
I'm not exactly sure how AP courses are graded, but the equivalent in the UK (called A Levels) are based on a single set of exams at the end of a two year course of study in the final year at high school.
There is no equivalent of the SAT in the UK (apart from the specialist exams for law or medicine etc).
Students will normally apply through the British equivalent of the Common Application (here it is called UCAS) in the final year of high school.
Universities will then generally make "conditional" offers. What this means is that if the student gets the required grades in the upcoming exams then they will be accepted.
Required grades can vary greatly (as I'm sure that they do at US universities).
I used to work for a company that recruited people from certain UK universities that were very highly rated.
One of these (Warwick) for the particular course that we recruited graduates from had a standard entry requirement of A star A star A.
They stated that the equivalent AP grades required for any US students wanting to come and study were five AP Tests at grade 5 including AP Calculus BC (and, very specifically, NOT AP Calculus AB). Would US universities insist on particular AP Test grades like this?
Of course, not all subjects have such high grade requirements.
Outside of those in demand courses, other courses might require something like 5, 5, 4, 4 in AP courses (AAB in A levels).
There is a page here about US entry requirements at Warwick:
https://warwick.ac.uk/study/international/admissions/entry-requirements/usa
.
At the other end of the scale, there is a university near me (I won't name it) where the entry requirements for US students are as follows:
- High School Graduation Diploma Grade B/ CGPA 3.0 or above plus one of the following: Scholastic Aptitude Test (SAT 1), administered beginning spring 2016: a minimum of 610 in Evidence-Based Reading and Writing and a minimum of 570 in Maths; 2 Advanced Placement Tests (APs) in subjects relevant to the study area with grade 3; ACT (American College Test) with a composite score of 23 or above.