Android/Pin App

You can pin an app's screen to keep it in view until you unpin it.

For example, you can pin an app and hand your phone to a friend. With the screen pinned, your friend can use only that app. To use your other apps again, you can unpin the screen.

https://support.google.com/android/answer/6118421?hl=en

Turn on screen pinning
 * 1) Open your device's Settings app Settings app.
 * 2) Tap Security & Location And then Advanced And then Screen pinning.
 * 3) Turn on Screen pinning.