I think you didn't create a flutter project. you created an android studio default project.
Create flutter project - File > New > New Flutter Project
If you didn't install flutter correctly, Here are the steps to install Flutter after installing Android Studio:
Download the Flutter SDK: You can download the latest version of the Flutter SDK from the official Flutter website.
Extract the Flutter SDK: Extract the contents of the downloaded ZIP file to a location on your computer.
Set up your environment variables
Verify your installation: Open a terminal window and run the command 'flutter doctor' to check that your installation is complete and there are no missing dependencies.
Install the Flutter plugin in Android Studio: Start Android Studio and go to the plugin marketplace. Search for the Flutter plugin and click the Install button. Restart Android Studio after the installation is complete.
Create a new Flutter project: Go to File > New > New Flutter Project
to create a new Flutter project.
Now, you can find main.dart
file and lib folder.
or
There is macOS install - Flutter documentation - here