Intertextuality is quite simply... When one film borrows an idea from another film (Simplest way to put it)
The long wayyy on the other hand is......'Intertextuality is a term to describe the visual referencing between films. Quite literally, films 'borrow' from each other, and you may recognise certain camera angles, aspects of mise en scene, snippets of sound or methods of editing in some films that you have seen in others.'
Intertextuality happens almost all the time in films, and we may not notice it because the ideas are borrowed from films that were out over 30 years ago, kinda films my grandparents watched.