Media queries
https://developer.mozilla.org/en-US/docs/Web/CSS/Media_Queries
Start by defining what should happen on small screen sizes first! Smaller screen sizes require simpler layouts. Then, adjust your styles to accommodate larger devices. If you design the complicated case first, then you have to work to try to make it simple again. source
Beginner's guide to media queries - https://developer.mozilla.org/en-US/docs/Learn/CSS/CSS_layout/Media_queries
Also see Responsive design - https://developer.mozilla.org/en-US/docs/Learn/CSS/CSS_layout/Responsive_Design
What Is Mobile First Design and Why It Is Important Today - https://www.mobileapps.com/blog/mobile-first-design
Breakpoints
https://material.io/design/layout/responsive-layout-grid.html#breakpoints
https://github.com/sw-yx/spark-joy#responsive-layout-breakpoints