Web Development Question

[Grace.]

Alright so, my question is how do sites like for example https://hide.me/en/proxy , etc display the site you input into it.

My first guess was an iframe, or something of the sorts , however, they're able to query and display pretty much any site, something that isn't possible with pretty much any method I was able to find when I searched it up, thinking perhaps someone had already asked this question.
( As they display any site, but a large amount of sites are set to same-origin only )

 

[itslyn]

Can't you hack iframes to do this in some ways?

 

[Xenons]

I might be completely wrong on this but from just a quick glance I saw that one of the first things that are loaded once you go to a site from that hide.me is
https://nl.hideproxy.me/go.php?u=A7...0VV05yuRbv5NafYyXcB/6g2BzFz5ZHaHXe8ALxNPN&b=5

My quick assumption is that it takes the files and adds additional code
or adds the script like you would through inspect element but before you actually load the page and send all the info to it