Which is the right place to place close button in a mobile app?

1. Where should close button be?
2. What is the best button placement for mobile?
3. Should primary button be on top or bottom?

Where should close button be?

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

What is the best button placement for mobile?

Optimal button placement follows the Gutenberg Principle. You should place your buttons at the end of the user's scanning path when they're ready to take action. There are rare cases when users are ready to take action before scanning, but this is only when they're already familiar with the screen content.

Should primary button be on top or bottom?

In small windows, the primary action button should be placed at the bottom right. Our brains always scan the content on the screen in front of us, before we even realize it. On smaller windows it's best practice to have the most important action at the end. Hence, bottom, right.