Reference video: https://www.youtube.com/watch?v=_uOgXpEHNbc
Video source code: https://github.com/abuanwar072/Flutter-Responsive-Admin-Panel-or-Dashboard
Screenshots source code (site owner coded along with the video): https://github.com/dotnet9/FlutterTest/tree/main/src/admin_panel
This article does not intend to introduce the functional code. If you are interested, you can click on the video or source code above for reference.

Although there are many code files, each dart file actually has only a few dozen lines. The panel is divided into small module dart files, and finally combined for display.
First, take a look at the Mac desktop display (Windows desktop is similar):

Web version (press F12, debugging same as frontend):

iOS (iPhone 12 Pro Max simulator):
Android (emulator):
Original video recorded a short clip:
Are you interested? Go back to the beginning of the article and click the video (or source code) to learn.