In J3, we had a modal that went to a registration page using {modals url=/registration-page}. The code behind the registration menu item redirects the user to a new page. This works well in J3.
But with Modals moving to an iFrame model in J4, the redirect is displayed within the modal iFrame rather than back on the main window.
You can't do that in the {modal} tag.
You could do this by creating a template override for the registration page and add the target to the form element.
The functionality to load pages inline is not included in the Joomla 4 version, as it caused too many issues.
So yes, with the Joomla 4 version, all urls are loaded via iframes.
You can still do what you need with a little html knowledge. If that is too high-level for you, then either look for someone who can do it for you, or look for a different solution than Modals.