One difference I see is that Oppia seems to be making AJAX calls (am not familiar with Angular, so, maybe I am wrong). Whereas, I deliberately chose the quiz flow to be one that goes to a different URL as you progress. But, yes, will take a look at the backend and DB and see how things are stored and processed.

I learnt just enough web development to be able to build this. So, it will take me a little bit of time to understand code written by a Google team :)