Hello videoslowmofunction,
Thank you for using Apple Support Communities.
We understand that you are entering a number on your iPhone using the phone keypad and are wondering how it recognizes that you are entering a 'G', 'H', or 'I' when you press the number '4' or 'T', 'U', or 'V' when pressing '8'. If the form or entry field you are filling out is set to recognize entries from a phone type keypad, it will depend on how they have it set up. They will sometimes only recognize a specific letter when you press a number key or you may press the key multiple times to cycle through the options. For example, if you want to enter the letter 'I', you would press the number '4' 3 times.
But with the number of smart phones, most forms or entry fields will allow you to use the keyboard and you can enter letters normally.
Best regards.