Thanks for the comment! So we just launched our Android version today: https://play.google.com/store/apps/details?id=com.py (it's still in alpha and there are a number of issues we've noticed that working around the clock to resolve ASAP)

RE personalization: At the end of each chapter, users answer quizzes. We motivate people to review wrong answers through a star system: a user will earn 3 stars if they answer 100% of the assessments correct, otherwise, they have can review to earn more. This allows us to customize the content that users see while also gamifying the experience to incentive people to spend more time on concepts they’re struggling with.