These examples use a fixed size display area. One frame can also be full screen so appears as if you are connected directly to the site BUT the URL does not change.
The window below is to the u3a WordPress development forum
This window is to SiteBuilder https://u3asites.org.uk/hammersmith-fulham/home
Now it seem that SiteBuilder detects the iframe and stops itself displaying.
This is to the SiteWorks website.