Singleton = one instance for the whole app (cache). Scoped = one per HTTP request (DbContext). Transient = new every time (lightweight helpers). In ShopNest, AppDbContext is Scoped so two requests never share the same DB tracker.
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.