Hey!
For the UI I designed all of it in Figma, its free and cloud based, so great to work with other devs too! I use a lot of image buttons and text labels, and figma makes it easier to grab the x and ypos as well as see how itll look layout wise!
The project is all renpy though! It's pretty straight forward for standard visual novels! Chatsims like this, or anything else with extra features, can get a bit confusing, but the community is super helpful!