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 :)
"I learnt just enough web development to be able to build this."
Congrats! Seriously, this is the best way to learn. We're all on the same journey :)
EDIT: Yes, Oppia doesn't appear to update the URL after each step. So if you refresh the page you're back at the start of the lesson. (They didn't have to do it that way, though: http://www.codemag.com/article/1301091)
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 :)