The more granular decomposition is something I built separately. I take a standard/topic and break it down into coherent concepts, then into the smallest independently useful retrieval skills I want to track. For Mathy that's especially constrained because I only care about things that make sense for automaticity, not everything you'd need to teach the subject.
So for example a standard might imply multiplication/division fluency, but internally that gets decomposed into individual mathematical relationships and retrieval directions. `7 × 8`, `56 ÷ 7`, and `56 ÷ 8` are related, but they're not necessarily the same learning state.
Then for procedural material I define bounded problem families rather than copying individual questions. A family specifies the operand domain, mathematical rule, intended strategy, answer form, exclusions, hardest cases, etc, and the questions are generated deterministically from that.
So unfortunately I don't have a magical curriculum-generation hack :) It's been a combination of standards/course outlines for scope + a lot of decomposition + AI helping propose/check things + deterministic validation afterward. It was a lot of experimentation, and I still have room to improve.
I also intentionally don't copy questions from textbooks. The standards and public course outlines tell me what belongs where, but the actual problems/content are independently authored or generated. That makes it much easier to reason about correctness, coverage, etc.
For the domaine, yea, it's important. I built-in steaks and XP/levels since I could borrow the general approach from a previous habit tracking app I made (maincharacter.game). I also added medals for achievements. The website version has a leaderboard, which I may also add to the iOS app, but would take some add'l work since it's currently offline only ATM.