Working with flexbox won’t just be applying a class, it will require creating new classes that use some flexbox sass mixins we have(easier than remembering browser prefixes, etc).
We have a page template page-flexbox.php for showcasing how we would typically do some flexbox based layouts
The styling for this flexbox template is in pages.scss but should be removed from production when you start working on your actual styling in pages.scss
If you aren’t sure which mixins to use take a look at the _flexbox.scss in the starter themes scss folder.
Here are a few other resources I found helpful: