When clicking on a button in your website to open a Popup, the page performs an Ajax request to your server to fetch the content of the requested Popup, and after receving the result from the server, the content will be added into the Popup and it will be displayed to the user, hence the content of the Popup is added dynamically into the page after the user requests opening the Popup (the content of the popup is not loaded when the page loads).
When the Popup opens the elements of the Theme will work normally and the content of other plugins (for example Elementor, Contact Form 7, Quform) will be initialized and their javascript elements will work normally in the Popup, however, if you are using other third party plugin that doesn’t support loading its elements dynamically (adding the content dynamically into the page after that page is loaded), we recommend you to check the plugin documentation (or contact their support) to see if the plugin supports using and initializing its element dynamically, and if they do and provide a javascript function to inilize their elements dynamically you can add it inside this code snippet then add it into Theme options > Layout > Advanced > Custom JS (in footer):
$('body').on('pix_popup_open', function(){
// Your code here
});