I'm having trouble comprehending what's needed in terms of the presentation format of my AMP pages.

My site happens to be a Drupal site.
The Drupal AMP project comes with an example theme.

1) In its CSS file, the default value for the width of the an AMP page's content is 620px.

2) The CSS file itself contains absolutely no media queries (as in no adjustments are being made for different screen sizes).

I get that AMP is intended for mobile. But also realize that an AMP page could be viewed on any type of device. So I don't understand what I'm supposed to do.

Am I supposed to be creating a format that adjusts to different screen sizes, like I would with a normal responsive site?

Or is there something else that's going on here with AMP that I don't have a comprehension of that somehow just automatically takes care of this issue for me?

1) Does setting a max-width down around 620px make a good plan, or should it be wider as I have done with the responsive version of this website?

2) Am I supposed to be using media query breakpoints for different sized screens?