The app icon is shown on the device's home screen, in different parts of the operating system and in App Store/Google Play.
The launch screen (sometimes called splashscreen) is a full-screen image that appears immediately when your app starts up. It is Apple's intention that the launch screen helps make your app feel fast and responsive. As such, it should represent static parts of your app's initial UI, so that when the app actually launches, the transition will be seamless.
That said, many apps show a logo or similar on the launch screen.
Composer only supports static PNG images for the launch screen. This means a separate image is required for all supported screen sizes.
The image will be centered/cropped based on the orientation and screen size, but not beyond the red max boundary area.
You can read more about the launch screen and view a list of required screen sizes in Apple's Launch Screen documentation and Google's Material Design launch screen documentation.
Permissions for iOS builds are configured automatically, but you should check and modify the usage strings according to how your app makes use of the requested permissions.
For Android you can manually choose permissions, but the required ones are highlighted and included by default.
Using some components or flow functions may require additional settings, such as API keys. They can be configure under the Advanced tab.