i dont see how to close an app on the IPhone Mini 12

You shouldn’t close an app after use; it slows down your phone and increases battery drain, and in some instances it can result in data loss for the app closed. See→Do not close iPhone and iPad 'background'… - Apple Community.

But if you want to go against the advice of experts, swipe up from the bottom of the screen and pause mid-screen.

Hello!

To close an app after use just swipe up from the bottom center off the screen. There is a little bar at the bottom showing where to swipe up from.