Android Launch Screen
Android launch screen and App Icons are placed in /android/app/src/main/res the launch screen logo is in drawable directory inside the res directory. Please make sure you name the file logo.png
The launch screen layout is /android/app/src/main/res/layout/launch_screen.xml
IOS Launch Screen
The IOS launch storyboard is designed in /ios/LaunchScreen.storyboard. You can customize the storyboard in Xcode according to your requirements.
App Icons
You can easily generate app icons using online generators of your own choice or use https://appicon.co/ for quick and easy approach.
Download the generated icons and replace in the respective directories mipmap-{x} where the {x} donates a resolution.