I'd have to first of all start by saying, there's no perfect 
approach to having the best Flutter codebase. Phew!!!, with that 
out of the way, here's a less complicated and easy to use approach 
to structuring your Flutter Codebase.

...
1. In your VsCode, click on the 'double-file-like' icon, to 
   reveal your side bar.
2. Select the 'Square-like' icon, that has it's first quadrant 
   apart to open your 'Extension market' or click 'Ctrl+shift+X'.
3. On the search bar, enter Dart, select the first extension that 
   pops up, and install.
4. Since I have already installed mine, it's showing 'Uninstall',       
   but yours (if you haven't installed it yet), will have the 
   'install' text instead.

Visit the Flutter official page, to learn how to create a new 
application, as that is not our major concern in this article.

...


Components

This is found at the level of the main.dart file, which 

provides necessary re-usable components that should be 

universally available anywhere in the application, e.g, the 

likes of action button, etc.
Model

This controls the structure of data you wish to use. For 

instance, say you want to create a blog post, the title, name 

of author, content, created_at, updated_at, etc, would form 

the model of the blog.dart class.
Providers

If you are using the flutter provider package, this would 

enable you split your provider helpers.
Screens

This literally means the pages of your application.
Home_screen

Lifting just the 'home_screen' as an example, it's cleaner to 

have a folder of each screen/page you wish to implement, this 

gives you the access to have other elements within the 

home_screen file.
Components

This components, does the same this as the No.1, but it's 

access is only limited to the particular screen you're working 

on. You could see the 'body.dart', which would literally 

contain the body of the scaffold in the home_screen.dart.
Enums

In flutter/dart, enums, would help you establish small set of 

predefined set of values that will be used across the 

application logic.
SizeConfig

This comes in handy, when you wish to outline sets of 

functions, that would help calculate and return, on the fly, 

the width and height of the device you're currently working 

on.
Theme

This works like the themeData in the scaffold. Here you get to 

declare some variables and User Interface settings. Some call 

it constants.