If its a web based platform such as SquareSpace, WordPress or Concrete 5 then you will generally get a website that works with both a desktop/laptop and a mobile browser - this is called a Responsive Design as the site works out how big yoiu screen is and then changes the layout accordingly.
This is not the same as creating an app that can be installed via the app store but you can add it to the home screen and it will look like an app.
Creating an app, also called a Native Application, requires the application to be written in Java (android) or Objective C (iPhone) so its not easy to created one app that works on both phones.
I'm thinking along the lines of creating a calculator or graph creating type thing where you input data and it stores, compares and displays it <vague>
So no, not web based. There are a fair few tools out, usually pay, which promise to allow you to create apps, but when I was looking at reviews it wasn't really clear if they were for standalone apps, or like you say, for just scraping content from a Wordpress site or similar.
I have a couple of ideas that I'd like to play with, but I also have a total blind spot when it comes to coding and can't get my head around it at all, so was basically just wondering if any of the app creation tools are worth paying out for or if they don't do what I want them to do anyway.