I need to know whether Flutter will render the iOS Cupertino style on iOS and Material on Android with a single codebase. I want to know this before starting to develop my app w
These capabilities are under development. I suggest to look into this library: https://pub.dartlang.org/packages/flutter_platform_widgets
Other than that, there is a lot you can do already, even without this library. Everything depends on your resources, requirements and complexity of your project.
For example, if you are working alone or in a small team, your project's complexity is probably not overwhelming, and your entire application will consist only of a couple of dozens of Widget derived classes at max. In this case, maybe you can deliberately program the platform-adaptive behavior in your custom Widget classes, so that you'll be happy working with them long-term.
This workflow is not always applicable, and may seem complicated at the beginning, but it will become more natural later.