iPhone
iPhone 8 or earlier
- Double-press the Home button. This starts the app switcher, where you should see all of your apps represented as cards.
- Swipe side-to-side through your apps until you find the one you want to close.
- Swipe up on the app you want to close. When it disappears off the top of the screen, it’s closed.
Full instructions here (scroll to second half of page)
iPhone X and later (or running iOS 12 or later)
- Swipe up from the bottom of the screen and pause for a little moment until the app card pops up. In the app card, you will see all the apps which are currently residing inside the memory.
- Swipe left or right to move to the app card which you want to hard close.
Swipe the card up to hard close it.
Full instructions here
Android
- Close one app: Swipe up from the bottom, hold, then let go. Swipe up on the app.
- Close all apps: Swipe up from the bottom, hold, then let go. Swipe from left to right. On the left, tap Clear all.
- Close all apps on Android Go: Swipe up from the bottom, hold, and let go. At the bottom, tap Clear all.
- Find your Home screen: Tap Home
or Home . - Stop apps working in the background:
- To open Quick Settings, from the top of the screen, swipe down twice.
- To see the number of active apps running in the background:
- At the bottom left, tap # active apps.
- Or, at the bottom right, tap the number next to Settings and Power .
- To close each active app, tap Stop.
or Home 
.
and Power 
.
Full instructions here
