Guide to new Flexbox Container & Inner Container layout

Hallo, i usually create many hooks with html and i like to create sections with this way .

Before with Unsemantic i was working with grid-container classes etc. to work my custom html elements and set the default widths from the customizer easily to them.

Now lets say we need to create a section that

1. Takes the default width of the customizer ex. 1200px.

What classes should I use ?

<section class="container"> // Container of the default customizer //
<div class="inner-container"> // Inner Container //

// Flexbox Elements Childs //

</div>
</section>

And if there is any documentation uploaded for the new classes ?