That's basically what they do. Most apps like Google authenticator use totp rather than hotp. The t stands for time, the h is hmac. Except they also use an algo

That's basically what they do. Most apps like Google authenticator use totp rather than hotp. The t stands for time, the h is hmac. Except they also use an algorithm to convert those characters to digits only. It's all described under the totp section.