Aside the settings for each platform, you can customize the texts showed on the splash screen.
See FPlatformSplash::SetSplashText, the types are listed in SplashTextType::Type.
There is also SetCustomSplashImage which sets the GCustomSplashScreenFileName which is used in FGenericPlatformSplash::GetSplashPath.
Related types:
FPlatformSplashFGenericPlatformSplash