flutter/packages/flutter_tools/templates/README.md

This directory contains templates for flutter create.

The app_shared subdirectory is special. It provides files for all app templates (as opposed to plugin or module templates). As of May 2021, there are two app templates: app (the counter app) and skeleton (the more advanced list view/detail view app).

  ┌────────────┐
  │ app_shared │
  └──┬──────┬──┘
     │      │
     │      │
     ▼      ▼
┌─────┐    ┌──────────┐
│ app │    │ skeleton │
└─────┘    └──────────┘

Thanks to app_shared, the templates for app and skeleton can contain only the files that are specific to them alone, and the rest is automatically kept in sync.