When we talk about app prototypes, do we mean a fully functional application including databases and all functionality of the app? Or do we just mean the visual UI interacting through buttons etc but without any backend functionality to support it?
Long answer: In my experience, prototype is used very loosely and can mean many things. But the point is that a prototype is not a fully functioning app/system/software yet. It is used to demonstrate the possible features/functionalities that the app would/could have. How you demontrate it depends on the specific case. Some prototypes can be created using a simple word/powerpoint slide, some can be created using mockup screens/wireframes and some are semi-functioning products. I do not think there is one single answer to this.