Here's how to get rid of a split screen on iPad
and turn off the feature permanently.
If two apps are side by side with a divider between them,
drag this bar here all the way to the right or left.
If you drag it to the right,
you'll close the app on the right.
If you drag it all the way to the left,
you'll close the app on the left.
In both cases,
the remaining app will then become full screen.
If the second app is floating on top of the main app,
tap and hold the top of the floating app
and drag it downwards.
It'll snap to the right of the screen,
at which point it can be closed
by dragging the divider like this.
Swipe it to the right to close it temporarily,
and swipe left on the right side of the screen
to bring it back.
You can disable the split-screen function permanently
by going to settings.
Tap "Home Screen & Dock,"
and then "Multitasking."
Turn off "Allow Multiple Apps"
by sliding the switch to the left.