Apple Support Document: https://support.apple.com/en-us/HT201330


How to force an app to close on your iPhone, iPad, or iPod touch

If an app won’t respond, you can force it to close.

Force close an app

  1. On an iPhone X or later, or an iPad with iOS 12 or later, or iPadOS, from the Home screen, swipe up from the bottom of the screen and pause slightly in the middle of the screen. On an iPhone 8 or earlier, double-click the Home button to show your most recently used apps
  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. 

When your recently used apps appear, the apps aren’t open, but they're in standby mode to help you navigate and multitask. You should force an app to close only if it’s unresponsive.