Q:

What does cordova prepare do? 

I checked the documentation but didn't find a clear explanation about this command.  
So does anyone know what exactly does the command cordova prepare do? 

Does it update the platform-specific www folders? 

If yes, does it copy the full content of the root www? 

Does it update the platform-specific icons? 

If yes, where should the source icons be located? 

Does it update the platform-specific splash screens? 

If yes, where should the source splash screens be located? 

Does it update other platform-specific resources? 
Does it update the Android manifest? 

If yes, how can I customize the manifest, so that cordova prepare merges both the customization and the generated lines? 

Which other files are updated for the Android platform? 
Which other files are updated for the iOS platform? 

A:

Cordova prepare: It prepares [<platform> [..]] by copying the necessary files into the specified platform folder, or for all platform folders.  The project is then ready to be built by Eclipse, Xcode, etc. 
Refer to: Cordova Commands
As documentation says:

In this case, once you run prepare, you can use Apple's Xcode SDK as
  an alternative to modify and compile the platform-specific code that
  Cordova generates within platforms/ios.  You can use the same approach
  with other platforms' SDKs. 

Refer : Apache Cordova API Documentation

