how do I close my pandora app on an iphone 12?

See the following from Close an app on your iPhone or iPod touch - Apple Support

How to close an app

You should close an app only if it’s unresponsive. The way you close an app depends on the type of device that you have. Find your iPhone or iPod touch model below, then follow the steps. Or learn how to close an app on your iPad.

iPhone X and later

1. From the Home Screen, swipe up from the bottom of the screen and pause in the middle of the screen.
2. Swipe right or left to find the app that you want to close. 
3. Swipe up on the app's preview to close the app.