Responsive design is a method of using media queries in Cascading Style Sheets (CSS) to ensure a site is usable on various devices with different screen sizes, from very small to very large.
Full Stack Python uses responsive design to improve readability across a broad range of devices that people use to read this site.
This site is entirely on responsive design and has many articles on how to achieve readability across devices.
Using Media Queries For Responsive Design In 2018 contains a bunch of great examples for how to use media queries to achieve a responsive design.
Smashing Magazine's article from 2011 on responsive design - what it is and how to use it remains a definitive source for understanding why sites should be responsive.
Fundamentals of responsive images shows the code behind several ways to make your web images resize according to browser window size.