product.py

@strawberry.federation.type(keys=["id"], extend=True)
class UserType:
    id: strawberry.ID =

@strawberry.federation.type(keys=["id"], description="User Type definition")
class ProductType:
    id: strawberry.ID
    name: str

    @strawberry.field
    def created_by(self) -> UserType:

        return UserType(id=self.created_by)

###...

schema = strawberry.federation.Schema(query=Query, types=[UserType, ProductType], mutation=Mutation, enable_federation_2=True)

user.py

@strawberry.federation.type(keys=["id"])
class UserType:
    id: strawberry.ID
    name: str

    @classmethod
    def resolve_reference(cls, id: strawberry.ID):

        user = db.session.query(User, id)

        return user
      
### ...

schema = strawberry.federation.Schema(query=Query, types=[UserType], mutation=Mutation, enable_federation_2=True)