Embed external site into page and interact with content

I'm trying to load an external website on a page and interact with the content, for example add event listeners (for the content of the external site loaded). This won't work on an iframe as I can't interact with the content, I tried to do it using react, and when I want to add event listeners to the iframe I get the following js error Uncaught DOMException: Blocked a frame with origin "http://localhost:3000" from accessing a cross-origin frame. I thought it was just not possible but then I saw this video from a tool that seems to do that: https://youtu.be/qinZviJajcw?t=21

Does anyone have an idea on have this could be accomplished?

thanks!