No you dont have to use bootstrap again for flexbox. You can use Flexbox with out bootstrap. bootstrap have inbuilt flexbox classes, where as flexbox is just css property can be use alone with out bootstrap.

@mustafa-synectiks

excuse me with the delay for the reaction; but my real concern is that I have finished my e-commerce site with python / django my user interface is not too professional and I have made it done with bootstrap, jquery. And my concern is that I would like to make it pro like that of alibaba or amazon (with reactive basket system and many others) thank you for all attempted response!

thank you

This image is not quite accurate, flexbox is not a framework. And what does flexibility is a polyfill for Flexbox mean.