RolepaperFor frontend: build a UI from a spec, talk about accessibility, and explain a performance trace. For backend: design a small API, talk about indexes, and describe a production incident.
For electrical and embedded: a circuit you can draw, a firmware bug you actually found, and how you test on hardware you cannot rewind.
System design at junior level is “how would you store this and what fails.” At senior level it is trade-offs and cost. Rehearse out loud. An AI interviewer is optional. A rubber duck is enough.