One page, different layouts
On a laptop, a shop might show products in four columns; on a phone, it might show them in one. The same content rearranges so it remains easy to read and tap.
Designers use breakpoints, screen widths where the layout changes. A fluid layout also stretches or shrinks between those widths.
More than phone versus desktop
Mobile-first means designing for small screens first, then adapting the layout for larger ones. Responsive design also accounts for zoom, larger text, resized windows, and device rotation so the page remains usable. Supporting zoom and larger text also helps with accessibility.