These are Cordova resources. You can replace icon.png and splash.png and run ionic cordova resources to generate custom icons and splash screens for your app. See ionic cordova resources --help for details.

Cordova reference documentation:

• Icons: https://cordova.apache.org/docs/en/latest/config_ref/images.html
• Splash Screens: https://cordova.apache.org/docs/en/latest/reference/cordova-plugin-splashscreen/