In particular, $\big(f(\Box)\big)'= f'(\Box) \cdot \Box'$, and we can imagine to put whatever other function inside the box. (What happens if we put $x$ in it?)
Example: Compute the derivative of $y=\sin(x^2)$.
Solution:
We take $g(x)=x^2$
and $f(x)=\sin(x)$, so that $f(g(x))=\sin(x^2)$.
Since the derivative of $x^2$ is $2x$ and the derivative of
sine is cosine,
$$\frac{d }{dx}\left(\sin(x^2)\right) = \cos(x^2)\cdot (x^2)' = \cos(x^2)\cdot 2x.$$