With an iFrame you can embed external documents or applications. You do this using a URL or link, which is then displayed in the iFrame element.
Examples of use include audio and video players (Soundcloud, Twitch, Dailymotion, etc.), a blog, appointment calendar, booking form or currency converter. A PDF can also be embedded in a roundabout way using an iFrame (see here). Of course, there are countless other possibilities.
Here you can read how to insert an iFrame.
Here is a website in Website Creator, with examples of how to use iFrames.
Whether you can embed an app using iFrame depends on the app provider. Some providers may have security policies that prohibit embedding on external platforms. Therefore, they will not appear in your iFrame after you paste their URL. Since this is controlled by the external coding, it is not possible to control or bypass this via the homepage builder .