The command to add desktop support are those one:
flutter config --enable-windows-desktop
flutter config --enable-macos-desktop
flutter config --enable-linux-desktop
Once you used them, you change your flutter config.
Then, the usage of
flutter create . simply mean "Create a new flutter project in my current folder".
If you're already in a Flutter project, it will just "recreate" your project by adding the new support for Desktop since you specify in your flutter config.