This post is not about the main concepts of CSS Flexbox. There are a lot of excelent posts that do it, for example: https://css-tricks.com/snippets/css/a-guide-to-flexbox/
I want to share a Flexbox Simulator that I've done.
It is very simple and you can understand quickly how to build your CSS Flexbox Container according to your needs.
The main idea is to modify the flexbox-container inputs and see how the flexbox-items react.
Hope you enjoy it!