How is this page moving as one?

HTML-CSS
1

This codepen All my webpages squash together, I like how the middle of this page moves freely as one. I can’t identify what it is in the code.

Cheers

2

I don’t understand what effect you want, can you explain more?

3

As you narrow the page from the left/right, the box as a whole moves with the page.

Whenever I have a box of some sort on my page and narrow the width of the page, one side of the box sticks and the other just ‘squashes’ rather than keeping the same size/shape.

[edit] I see it’s just using flex-box with a container of 100% then just having the inner box move freely inside.