Interface = contract only (what). Abstract class = shared base + some code (how + what). ShopNest: IPaymentGateway is an interface; PaymentBase abstract class can hold shared logging for UPI and Card.
Siemens rounds often probe: Industrial software.
Themes commonly reported in public MNC interview experiences (AmbitionBox / LinkedIn / Glassdoor-style). Practice aloud; keep answers this short in the real round.