So while Nat4s and 5s are one year courses each with exams at the end of the year, GCSEs are two year courses in a similar range of subjects that you begin in Y10(=S3?) and take at the end of Y11/S4
Then A-Levels are the two year course afterwards (start in Y12/S5 and take exams at the end of Y13/S6) which roughly equates in a similar way to the Highers and Advanced Highers. Generally Highers are a similar standard to the first year of A-Level work (AS Levels) hence why English universities generally donβt accept Highers alone
Advanced Highers are often harder than the full A Level/second year of A-Level work so if you go to university in England you will probably be a bit ahead, however they are similarly matched.
For GCSEs, you generally take a similar number of subjects to Nat5s or a few more (I took 11 but generally people take about 9-12) and then at A Level the standard number of subjects taken is 3 but a number take 4 and very few take 5 or 6
For GCSE it is generally compulsory to take two English exams (Language is comprehension and creative/persuasive writing, Literature is analysing texts), Maths and usually all three sciences in some way, although this isnβt compulsory