How to turn off horizontal split screen on iOS 14 on an iPhone?

I keep accidentally splitting the screen horizontally - which has led to zoom call issues etc. However, despite the split, I can't seem to figure out how to get 2 apps to display on my iPhone. So, how do I make sure the screen does not split? (Which gesture even causes the split? I seem to be able to do the split only randomly.) A quick google search only yields iPad tutorials** - the screen settings differ on an iPhone.