C’s to A’s in A level genuine question

I always see so many people saying that it’s possible to go from B/C to A/A* in A level if you work hard enough.

what i’m confused about which they never mention is whether they keep those AS grades they got and somehow balance it out with A2 in their final A levels or do they retake their AS after learning how to study more efficiently which would make it more realistic?

I got BBC for AS and I need to know how realistic is it to have at least 2 A’s in my final A levels diploma.