I don’t know how it works, but I don’t think it is “shared”. I think people (who consent in advance) will be notified when DNA matches are identified among the subscribers.
I have no idea whether these services (Ancestry.com, 23 and Me, whatever others) share DNA results among each other? Or limit their matching results to only their own customers.
Anestry customers are matched with other ancestry customers, so on for each of the companies.