The application icon, meaning the image used to represent the application in the OS and used as the icon for its main window, is stored in src/appicon. It is bundled into the application. You can access it from the code using the resource path :/appicon/sc-apps-{APP_NAME}.svg.
For a better integration with Linux desktops, icons used to represent actions should be loaded using QIcon::fromTheme(name). Unfortunately, Windows and macOS have no notion of icon themes, so you have to provide your own icons for them. To do so, copy the required icons to the src/icons directory and add them to src/app.qrc. The generated code in main.cpp takes care of ensuring QIcon::fromTheme(name) will load icons from this directory on Windows and macOS, without having to manually declare a fallback icon.