The game uses Ren'Py as its engine. As for the visuals, they are created using StudioNEOV2 from Honey Select 2.
Loving the game, still on chapter 3 (just had that amazing date scene in the forest) and can't believe chapter 4 is just out! I saw that HS2 is available on steam but I think you don't get Studio Neo with it via that route? Can you suggest how I would obtain Studio Neo V2 as I couldn't figure it out?
StudioNeo is a tool developed by Illusion (the creators of Honey Select) that's integrated into all their games and allows you to make artistic content with the characters you've created. However, in its original state, it's very basic, and you'll need many mods to make it truly complete. I recommend BetterRepack by ScrewThisNoise, which is a great starting point.