Well, first, I definitely wouldn’t store a list of questions on the User data type, and I don’t understand the mock data types at all. If a user is making a quiz by selecting questions from a question bank, I would almost certainly have a Quiz data type that stores a list of questions associated with the quiz.
You have a lot going on there, and honestly, I pretty much always steer clear of threads where someone needs help at the “I don’t really know where to start with my data types” level because that’s more than I want to get into in the forum. I know there are other folks who like to jump in and help at that level, though, so maybe one of them will provide some thoughts here.
Oh, and it couldn’t hurt to check out stuff like this blog post and this forum post, and I’m sure there are other resources out there if you do some searching.