I have some confusion in Munkres topology . My confusion is given below marked in red colour

[![enter image description here](https://i.stack.imgur.com/8X4tr.png)](https://i.stack.imgur.com/8X4tr.png)

**My attempt** : Here ${\bigcup}\_{x\in X} T\_x = \bigcup(X \times b) \cup (x \times Y)= \bigcup (X \cup Y)$

**My doubt** : why $ \bigcup (X \cup Y)= X\times Y $?