I am doing biology a level so can explain it in terms of that. My exam board is OCR.
A levels are generally studied for 2 years. At the end of the second year 2 to 3 exams are taken, the majority of A levels are fully exam based now, there are a few that still do coursework but not many.
My biology is split into 3 exams, aqa is probably the same. Each exam has set modules within it, these can be found on the specification which will be on the exam boards website. The spec will honestly be your best friend when it comes to revision as this shows you exactly what you need to know and in how much detail. there’s also past papers on your exam boards website which you should use as these are really helpful for revision.
when you’re learning your a levels the first few topics are generally quite basic and lead nicely on from gcses (do you do gcses?) for example the first module in my biology a level is foundations in biology which teaches about cells, biological molecules etc. this information is useful when learning other parts of the course later on. A general rule is that the content gets progressively more detailed throughout the course.
i would definitely recommend buying revision guides for your course too. if you have any questions let me know!