How could macOS installer act as both an installer package "InstallAssistant.pkg" and a disk image "SharedSupport.dmg"?

Apple has been providing single file macOS installer package "InstallAssistant.pkg" since Big Sur, this package install the app to "/Applications/Install macOS Ventura.app", and copy itself to "/Applications/Install macOS Ventura.app/Contents/SharedSupport/SharedSupport.dmg" using /bin/cp command. My Question is, How could this file act as both an installer package and a disk image? You can get the installer url from flowing page: https://mrmacintosh.com/macos-ventura-13-full-installer-database-download-directly-from-apple/