Extra Ue personal data

Hello; I'm a little bit confused about this scenario in terms of GDPR application: the company A owns the company B, wich is located in USA; the company B serves not only lot of USA B2B customers called D1, D2, D3,....but these B2B customers offer a "tailored" service for final customers C1, C2,C3,...... In other words, Ci are customers of Di but NOT direct customers of B. All customers are in USA (B, C, D). The data collected are processed by the mother company in Italy A that, finally, delivers the product to B and, finally, B will deliver to D wich will deliver to C. Suppose the data are quite sensitive because we are working about tailored suits. Is not clear in this scenario the roles of processors, controller and the role of information about personal data. Kind regards and thank you. Bruno