Let's say user A has received different milestones. Does StackOverflow have any ways of identifying that this user A now belongs to an expert community for this specific subject?
No, there's nothing that officially identifies a user as an expert in a subject on Stack Overflow. The closest thing we have is the tag badges: Bronze, Silver, and Gold. These merely indicate that a user has gotten X upvotes on Y posts in that tag (see the links for actual values of X and Y for each badge).
They'd have Bronze, Silver and Gold tag badges for the topics they were answering in, depending on the votes they'd received in those tags.
See here about half way down.