Keyboard switching icon
Hold down on the keyboard switching icon (usually a globe or smiling face) and on the last menu item, pick the right or left option. Note that this only works when more than one keyboard is enabled because the keyboard switching icon only shows when needed.
The 3 keyboard icons are for enabling the one-handed layout.
If you pick the left option, you'll be able to use the keyboard with only your left hand, and the same is true with the right option. The center option makes the keyboard two-handed again.
In the Keyboard settings (Settings -> General -> Keyboard -> One Handed Keyboard), you can select between the keyboards.
This method works regardless of how many keyboards you’ve enabled.
Here's an example of the right-handed keyboard: