How did Ancestry.com determine this? I doubt the individual used Ancestry so he’d be in the genetic database.
Seems like an invasion of privacy to reveal someone’s secrets like that.
It’s done by testing yourself and analyzing your matches. You take your highest matches with trees, reconstruct them using records in ancestry’s database, and looking for commonalities. It can be very painstaking, depending on how close your matches are and whether there are adequate record savailable. My search angel found my biological father with 4th cousin matches. I found my husband’s grandfather using social security death records and a 2nd cousin match. Then a match popped up, an adoptee looking for her bio parents. I was able to determine what family group her father was from.
You don’t have to test to be identified. It can be a match with a cousin you don’t even know.