As for X, if you want to customize the mobile menu functionality just like you mentioned. you would need to do more CSS and JS customization to the default mobile menu to achieve what you are aiming for.
But in case you want to find the PHP file that has the navigation/menu, it is the _nav-primary.php which you can find in *\wp-content\themes\x\framework\legacy\cranium\headers\views\global*.
Please make sure to use a child theme before making an customizations so that you will not lose any changes you did when you update the theme.
You can find more information about it here:
Hope this helps.