Two ways an app adjusts
Responsive design rearranges the same content to fit the space. On a phone held upright, product cards might stack in one column. Turn it sideways and they can sit side by side.
Adaptive design changes layout or controls for a situation. It might put important buttons within thumb reach on a phone, or add shortcuts when a keyboard is connected. An app can use both approaches.
Screen width is only one consideration
Rotation, foldable screens, larger text and accessibility settings can also affect what works well. Teams should test these situations and keep important tasks available on smaller screens.