The game is made in a program called Construct 3. I feel sort of bad saying this, but if you are looking for an engine to use I actually don’t recommend it. I think it works fine for smaller and probably more mobile focused games, but I feel that for larger projects with a PC focus like this it doesn’t hold up.
In any case I used Construct because it was what I knew when I started, but yeah I think there are better options out there. Apologies if that was more of an answer than you were looking for.