If you asked a 3rd or 4th year mathematician to "Write all you know about the area of ...." and named a common topic, they'd probably not even know where to start because you'll end up learning a considerable amount in university.
Bear in mind lecture notes for a single course can be anywhere from 50 to 200 A4 pages long, you're then supposed to 'join the dots' a lot in your head (no more spoon feeding like in A Level!) and in the cases of disseratations you're expected to read around the material, using textbooks and even published papers.
Wrangler and I had to write essays for our 4th year and I don't know anyone who stayed around the 5,000 word 'suggested length', everyone going to abotu 7,000 words or more. One friend even had to cut it back from somewhere in the region of 12,000~15,000 because it was longer than needed. If I'd not had to revise for exams and I'd had the drive I could have probably expanded it to 4 or 5 times it's size without even having to read more material, just put back in the results I skimmed over.
Even at university undergraduate level, where you're exposed to 100 times the number of results you are at A Level (per term I'd say!) if you take any 'interesting result' from a 3rd or 4th year course and go to the textbooks on it, you'll find that what is only 1 or 2 pages in your lecture notes has entire chapters devoted to it, sometimes entire books or series of books!
Things like matrices seem somewhat boring at A Level and I'm sure more than a few A Level students have asked themselves "WTF is the point in these". You could write 100 books on the applications of matrices to quantum mechanics, general relativity, group theory, symmetries, and 100 other topics which were too complicated and scary for me to want to learn, and you'd still have only scratched the surface of what matrices can be involved in.
On another forum I once read someone post "How can you do a degree in maths? Surely after A Level there's hardly any maths left to learn." Boy was he wrong. I could resit my 4th year 5 times and not make it to every lecture course they lecture, and those courses are each a tip of an enormous iceberg of material.