Why $y$ is a rational integer?

Question

I was reading that solution of $\mathbb{Z}[\sqrt{−n}]$ is not a UFD here Conclude that $\mathbb{Z}[\sqrt{−n}]$ is not a UFD.

But I do not understand this line "So either $d = 1, 2$ or $\gcd(y,\sqrt{-n}) \not = 1$ in which case $n|y$ because $y$ is a rational integer." Specifically, I do not understand the following:

1- why $d = 1, 2$ or $\gcd(y,\sqrt{-n}) \not = 1$?

2-and why in this case $n|y$?

3-and why $y$ is a rational integer?

Could someone explain this to me please?