Use a One Handed Keyboard
If the iPhone 8 or iPhone 8 Plus screen is too big for you to type one handed, you can set up a one-handed keyboard on the iPhone without using a third party keyboard.
Go to Settings -> General -> Keyboard -> One Handed Keyboard -> Left, Right, Off.
You'll need top pick where you want the keyboard to be shifted to when in one handed mode. You can also tap and hold on the globe on the keyboard and then tap on the left or right one handed keyboard option.