How do I get an underscore under a number or letter?

How to get the underscore will depend on the App you are using to type.

Apps like messages do not support underlining text, Apps like Notes, Pages, Word, etc... do support the underline as shown above.

Each App may have its now way to underlining text.

What App are you using exactly?

The underscore process is here on your keypad..

Select the entity you wish to underscore.

Then choose Underscore.