Setup
Suppose
$$ dZ_t=X_t\,dt+Y_t\,dB_t. $$Quadratic variation
Only the Brownian part contributes to quadratic variation, so
$$ d\langle Z\rangle_t=Y_t^2\,dt. $$Heuristic rule
Using differential notation,
$$ (dZ_t)^2=(X_t\,dt+Y_t\,dB_t)^2=Y_t^2\,dt, $$because
$$ (dt)^2=0, \qquad dt\,dB_t=0, \qquad (dB_t)^2=dt. $$