What you see upon launch is called Launch Screen or Splash Screen.
The default configuration allows you to showcase a static image upon launch, you may update the images for iOS at App_Resources/iOS/Assets.xcassets
Or Android at App_Resources/Android/src/main/res/drawable-***/background.png
On iOS, you may design your own customised launch screen if you are familiar with Storyboard, the default story board file may be located atApp_Resources/iOS/Assets.xcassets/LaunchScreen.storyboard
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If you like some sort of animation upon launch screen, checkout nativescript-splashscreen plugin.