Telegram bot: 
when user presses /start, the bot explains how it works - "the only thing you need is the URL of the YouTube video you want to extract the audio from. just text /getfile and follow instructions". the same works for /help 
principle of operation: 
the user presses /getfile, then bot asks to send him a link. and bot will respond by sending an mp3 file.

first, you need to develop a function that accepts a link from YouTube and downloads an mp3 file. I did it using the library youtube_dl. you can install it using "pip install youtube_dl"
then you need to create a bot. use this link https://python-telegram-bot.readthedocs.io/en/stable/ , there is detailed information about the bot in python. and after that you just need to attach your function here