/usr/sbin/installer The installer command is used to install macOS installer packages to a specified domain or volume. The installer command installs a single package per invocation, which is specified with the -package parameter.
/System/Library/CoreServices/Installer.app/Contents/MacOS/Installer Installer extracts and installs files out of .pkg packages, allowing developers to create uniform software installers. Installer launches when a package or metapackage file is opened. The installation process itself can vary substantially, as Installer allows developers to customize the information the user is presented with.