Which folder functions in ?

I’m customizing Traveryfy. But I can’t find function definitions.
For example action hook ‘travelify_before_main_container’, which folder can I find the add_action.

Right, now it works as placeholder and there is nothing assigned to this function.

You can assign anything you want and here is a quick example to just echo simple text. Wit similar manner you can add functions or whatever you want.

function random_function() {
   echo "This is a quick example that will output this text in places where travelify_before_main_container is executed";

add_action( 'travelify_before_main_container', 'random_function' );