record B (
  String value;
) {}

record A (
  Iterable<B> listOfB
) {}

class C {
  List<String> listOfB
}

@Mapper
interface Mapper {
  C mapAtoC(A input);

  default String mapBToSting(B in) {
    return in.value();
  }
}

interface MapperImpl {
  public C mapAtoC(A input) {
    ....

    C c = new C();
    
    List<String> listOfB = StreamSupport.stream(labels.spliterator(), false).map(this::mapBToSting).toList();
    c.setListOfB(listOfB);

    ....
  }
}