Salams, glad to hear you liked it! The game is open-source, so you can find the code on GitHub: https://github.com/deengames/eman-quest
The "TL;DR" version of the generation is that it's two stages: the first stage is to pick a bunch of random rooms on a grid (I think 9x9) and make sure they're connected. The second stage is to generate each room, as expected. (eg. the forest is a bunch of random points connected on a path, with some clearings; the castles are random interconnected rooms; and the caves are just a lot of random walk.)
If you're interested in discussing further, asking more questions, or just hanging out, you can find me on Discord quite often: https://discord.gg/frKXYtG