EDA software development

Hey guys, for people that have worked on developing EDA tools, I am curious about how the process looked like. I presume that the most common language is C++ that's why I'm posting this here Ate there any prominent architectures? Did you "consciously" think about patterns or did everything just come into place. How do you go on about developing the core logic such as simulation kernels? How coupled is the UI to the core logic? What are the hardest parts to deal with?

I would like to start working on a digital IC simulation tool (basically like LabVIEW for RTL) to learn a bit more of everything along the way and I'd love to hear advices from people with knowledge about it.