Most Kobo app development seems to be happening in two open-source projects - InkBox and Plato. Anyone can modify them and publish their work. A user added chess to a fork of Plato (
https://github.com/baskerville/plato/pull/281), but it likely won't make it into the main Plato as it's beyond the scope of what the lead developer wants. You'd have to install Rust and build it yourself if you want that chess app.
I also maintain a Plato fork that lets me mirror Firefox to my Libra 2, which I use daily. If you're looking to build a Kobo app, I'd suggest starting with Plato since it doesn't overwrite the Kobo OS. Plato uses Rust too, which in my experience makes development much smoother. The language helps ensure my code actually works before I install it on the device. InkBox is a more ambitious project that's open to big changes, but I'm too nervous to overwrite my Libra's OS to try it out.