Probably all over....
I took a look at the backside of some stock 328i rims I had in the backyard and all they say is BMW and the part number, not where they're actually made. BMW use to contract with BBS to make a lot of their rims but in the '90s and today they probably get them from several sources. They would still have to build to BMW specs though. Probably the same source that makes Audi, Volvo, Saab wheels could make them for BMW.