diff(x^2*y, y)

Let's perform the differentiation: $\frac{\mathrm{d}}{\mathrm{d}y}\left(x^{2}y\right)$

ⓘ When differentiating with respect to $y$, treat $x^2$ as constant. A constant factor stays in front of the derivative: that of $ku$ is $k·\mathrm{u\text{'}}$: $\frac{\mathrm{d}}{\mathrm{d}y}\left(x^{2}y\right)$ = $x^2·\frac{\mathrm{d}}{\mathrm{d}y}\left(y\right)$ = $x^2·1$ = $x^2$

▶Derivative:   $x^2$