Close button place depending on OS

1. Where should the close button be placed?
2. How can we use close button?

Where should the close button be placed?

The upper right corner of a modal window is the common place for the close button X in desktop experience.

How can we use close button?

(eXit button) Also called a "close" or "exit" button, clicking or tapping the X removes the current window, dialog box or popup message from the screen. It is also used to delete text and graphics.