Flutter is an open-source UI software development kit created by Google. It is used to develop cross-platform applications for Android, iOS, Linux, Mac, Windows, Google Fuchsia, and the web from a single codebase.

Flutter uses Dart, a programming language also developed by Google. Dart is optimized for building user interfaces and offers features such as a rich standard library, garbage collection, and strong typing.

It means to develop apps for multiple platforms (operating systems) or devices with one code base, at the same time, instead of building separate apps for each one. This way, one developer can for example develop mobile apps for Android and iOS. Sometimes it’s about compiling different versions of the same software for different operating systems and sometimes developers build one code to then make minor changes to it for each platform with the use of APIs. Generally, cross-platform app development enables the app to be made available to a wider audience in a shorter time. There are different methods of building such an app.

Flutter uses Dart for native-like performance, while React Native uses JavaScript and native components.

Flutter offers flexibility and expressive UI with the faster performance provided by Flutter’s code-Hot reload feature. A wide variety of users can get access and internationalization with the help of Flutter code to web applications. Flutter widgets come with native performance on both Android and iOS as Flutter code is compiled with ARM machine code using Dart's native compilers. Flutter provides faster deployment, customized designs, faster documentation, live and hot coding, and minimal coding with C/C++.

Flutter is different than most other options for building mobile apps because it doesn't rely on web browser technology nor the set of widgets that ship with each device. Instead, Flutter uses its own high-performance rendering engine to draw widgets. In addition, Flutter is different because it only has a thin layer of C/C++ code. Flutter implements most of its system (compositing, gestures, animation, framework, widgets, etc) in Dart (a modern, concise, object-oriented language) that developers can easily approach, read, change, replace, or remove. This gives developers tremendous control over the system, as well as significantly lowers the bar to approachability for the majority of the system.

There are many popular apps which use Flutter. Some of the apps are: - Reflectly - Google Ads - Alibaba - Tencent - Birch Finance - And many more. The use of Flutter in mobile applications is very high.

The widget tree is a hierarchical structure that represents the layout of a Flutter app. Each widget nests inside its parent widget, creating a tree-like structure that describes the user interface.

The pubspec.yaml file is a configuration file used by the Dart package manager, pub, to manage your Flutter project's dependencies and settings. It's where you define everything from the name of your project to the third-party packages you want to use in your app.

To upload to Play Store: - Google Play Developer Console - Link Developer Account with Google Wallet Merchant Account - Create Application - App Store Listing - Upload App Bundles or APK To Google Play - Time For Content Rating - Fix App Pricing and Distribution - Finally, Publish the Application To upload to App Store: - Creating an Apple ID - Signing up for the Apple Developer Program - Accessing App Store Connect - Getting Your App Ready for the Submission Process - Creating an App Store Listing - Upload the App Store Previews and Screenshots - Uploading Your App Using Xcode - Submitting Your App for Review.