I had a request to build a website for a targeted market that is allow its user to signup using their phone number only without entering email address just like mobile apps nowadays. I know this can be done with php framework such laravel + Firebase, but I'm just wondering, is this possible to be done with joomla 3.x or joomla 4.x maybe?
In the Joomla Extension Directory there is a free OTP plugin for Joomla available that uses an external commercial SMS (Text message) provider:
OTP Verification For Joomla
Maybe you can use the plugin + its SMS provider as solution, or use the code to develop your own Firebase OTP plugin?