Div with fixed width How to make responsive ?

Hello,

On the CLR course I learned that a fixed with with px is never good.
But now I want to try to make this challenge : https://www.frontendmentor.io/challenges/qr-code-component-iux_sIO_H

and it looks to me that the card has a fixed width on both mobile and desktop.

Is it good for this case to use a fixed width in px or can still beter use % and then a width and a max-width ?
Frontend Mentor
A perfect first challenge if you're new to HTML and CSS. The card layout doesn't shift, so it's ideal if you haven't learned about building responsive layouts yet.
Frontend Mentor | QR code component coding challenge
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