After you get the hang of the system, you can play with the interactive theorem prover behind it: Lean [2]. There's also plenty other interactive theorem provers (Coq, Isabelle, HOL, Mizar, Metamath, ...) but Lean has a lot of traction amongst mathematicians at the moment.
There are no limits to the math you an do with this. There is mathlib [3], the main mathematical library. It covers a lot of undergraduate material [4], and plenty of stuff beyond that [5]. The community has even covered some state of the art research math in Lean [6a, 6b].
You are very welcome to hang out on the leanprover zulip [7]] and ask questions about the Natural Number Game or anything else that is Lean-related.
[1]: https://wwwf.imperial.ac.uk/~buzzard/xena/natural_number_gam... [2]: https://leanprover-community.github.io/ [3]: https://github.com/leanprover-community/mathlib [4]: https://leanprover-community.github.io/undergrad.html [5]: https://leanprover-community.github.io/mathlib-overview.html [6a]: https://github.com/leanprover-community/lean-liquid [6b]: https://www.nature.com/articles/d41586-021-01627-2 [7]: https://leanprover.zulipchat.com/