How can I get an element by ID in a Custom HTML component?

I have the following HTML component in my Wix Website:

<html>
    <body> 
        <p>Execute a function when a user clicks on a button:</p>

        <button id="myBtn">Try it</button>

        <p id="demo">
        
        <script>
            const element = document.getElementById("myBtn");
            element.addEventListener("click", myFunction);
            function myFunction() {
                document.getElementById("demo").innerHTML = "Hello World";
             }
         </script>   
     </body>
</html>

This works without any problem. When I click the button, the paragraph text changes. My problem comes when I want to get an element of the site, oustide of this custom HTML component. For example the following code, prints NULL. When I render the page and I inspect it, I can clearly see that in the HTML DOM, exists a div with the id=‘SITE_CONTAINER’.

<html>
    <body> 
        <p>Execute a function when a user clicks on a button:</p>

        <button id="myBtn">Try it</button>
        
        <script>
            const element = document.getElementById("myBtn");
            element.addEventListener("click", myFunction);
            function myFunction() {
                const gottenElement = document.getElementById("SITE_CONTAINER")
                console.log(gottenElement)
             }
         </script>   
     </body>
</html>

In the HTML custom element I can only reference and get elements that I defined inside of it. Is there a way to access elements outside of this element, from this element?

All your investigations and conclusions are correct.

As long as you did not connect your HTML-Component with your Wix-Page by code, you won’t be able to communicate between these 2 elements (page/iFrame).

How to solve? ? ?
https://www.wix.com/velo/reference/$w/htmlcomponent/postmessage