John and Furnish bought their sons from California. The process here I believe has changed and parents are named as parents, not mother and father. This was, I believe, done so two men can be named as parents and not have to take on the title of mother, so if the birth certificates for their sons has Furnish named as the mother this needs updating. The original birth certificate will be on a register and 'sealed' and with have both the mother named as well as the father, whoever 'donated' the sperm and that was John, not furnish.
The boys can find their mother when they are older, it will be clear they had one of course. Furnish says she is a private person, so whilst much of their story is already in the public eye I doubt we will ever know if the boys have any contact with her or her own circumstances. It may not have been her egg, no details of the surrogacy arrangement were given. I do wonder how much she was paid.