The engine is Gamemaker Studios 2! I can't recommend it enough if you're looking to get into game dev- I'm by no means a genius and I learned it without too much hassle when I was super young without any game dev experience.

In the more simple dialogue trees, I just label numerically, then alphabetically if there's another branch after that. For a few interactions though I've got some truly disturbing spiderweb graphs that I need to straighten out.

I'm a developer who believes in quality over quantity- the game will almost certainly take less than 6 hours per run to make sure each interaction is as well realized as possible. My goal as a designer is to make something you play in a weekend, then talk about for 6 months. Not something that takes you 3 weeks and then you never think about again.