Weierstrass factorization theorem for sin Wikipedia says that
$$\sin \pi z = \pi z \prod_{n\neq 0} \left(1-\frac{z}{n}\right)e^{z/n} = \pi z\prod_{n=1}^\infty \left(1-\left(\frac{z}{n}\right)^2\right).$$
Where did the second equality come from?

The first product is absolutely convergent, so the order of the terms can be changed without changing its value. We pair the $\pm n$ terms, so
$$ \left( 1 - \frac{z}{n} \right)e^{z/n}\left( 1 - \frac{z}{-n} \right)e^{-z/n} = \left( 1 - \frac{z^2}{n^2} \right). $$
