Form submission handling outside of functions.php

Hello,

I have a lot of custom forms on my website and i would love to handle their submission outside of functions.php, i’ve read this article https://amp-wp.org/documentation/playbooks/handling-form-submissions/ and i managed to make successful request & response but my code was only working in functions.php, setting form action to another file or page doesn’t work. I would like to have something like my-amp-form-submissions.php and handle the amp form request there if this is possible.