Normally, the default iOS keyboard presents you with the alphabet. When you want to access the numbers, you just tap the “123” key, and then the numbers and symbols will appear. In order to revert to the letters, you have to then press the “ABC” key. There’s a faster way to type numbers, though.
Anyone familiar with Android devices knows you can press and hold keyboard letters to access numbers and symbols. iPhone users don’t have that kind of convenience, but this little trick is almost as good.