Hmm ... While all that might have been true at some point in the past, it seems to be a bit of a myth now. The particular boys who suit Winchester are the ones who took up an offer there, rather than Eton or Radley ...
I have it from the horse's mouth (former ten year old prep schooler) that those of his friends who went to Winchester in the past few years were no different, in brain or personality, to the boys who went elsewhere. Former ten year old also acknowledges that, while he had a different favourite amongst the small number of schools we applied to, he would have been perfectly happy at Winchester.
So - what sort of boy? Clever, independent, well organised. It's a boarding school, there would be no point being there unless you actually enjoy the company of your peers. And it could be miserable if you don't have a good amount of self-confidence and resilience. I can't think of anything particularly distinct that would mark out a 2021 Winchester boy as in any way different to other clever, fortunate schoolboys.