Every college will decide in their own way — some decide purely off your Year 12 end of year exams (whether they’re mocks or AS exams), some will decide purely on your work through the year, some will decide using a combination of both.
And whether your A at AS level becomes a predicted A* at A2 will often depend on the ‘strength’ of the As you’re receiving. eg I was a straight A student through AS levels and was often getting full marks in class & close to full marks in exams/my AS results so was predicted A*s in all subjects, but if I had been getting closer to the minimum marks for an A then it’s unlikely my teachers would have boosted me to an A* predicted at A2.
It’s always worth asking your teachers what they’re planning on predicting you/how they decide, and seeing if there’s any way you can improve if you’re unhappy with it!