For iOS:
1) On an iPhone X or later, iPad with iOS 12 or later, and iPadOS: From the Home screen, swipe up from the bottom of the screen and pause slightly in the middle of the screen. Then swipe up fully on the Chosen app in order to hard close it.
2) On an iPhone 8 or earlier: double-click the Home button to show your most recently used apps. Swipe right or left to find the app that you want to close. Swipe up on the app's preview to close the app.
For Android:
Go to Settings and then Apps.
Tap on the Chosen app.
Tap on the Stop or Force Stop button.