How to Work With a Statefull Widget?

Stateful widgets are stateless objects that can be used to do anything. A lot of times i try to abstract my business logic out, and then there's like a builder within there. It would be interesting to see the plethera of definitions that we get from around the inner net. And for all the people on the internet who talk about business logic this and that, i would love to see everyone do one vidia where they explain in their world, where's the line between business logic and whatever you don't consider business logic?