How should a Flutter web app's overall layout properly and gracefully adapt across a genuinely much wider range of possible screen sizes compared to a typical mobile app, spanning anywhere from a small mobile browser window all the way up to a genuinely very large desktop monitor?

A Flutter web app frequently needs to properly support a considerably wider practical range of possible screen sizes compared to a typical mobile app, since a website can genuinely be viewed on anything at all from a small mobile browser window up to a genuinely very large desktop monitor, and using LayoutBuilder together with carefully defined breakpoints, meaning specific screen width thresholds at which your layout meaningfully changes, such as switching from a single column layout on a narrow screen to a considerably more efficient multi column layout on a genuinely wider screen, helps ensure your particular web app's own layout remains genuinely usable and visually appealing across this entire considerably wider practical range of devices.

LayoutBuilder(
  builder: (context, constraints) {
    return constraints.maxWidth > 900
      ? ThreeColumnLayout()
      : SingleColumnLayout();
  },
)