The file oldsite/wp-admin/install.php is part of the WordPress installation process. Here are some detailed points regarding this file:
install.php file is responsible for setting up a new WordPress installation. It guides users through the process of configuring the database, setting up the site title, admin credentials, and other initial settings needed to get WordPress running.install.php played a more significant role in manual setup. Over time, WordPress has streamlined its installation process, making it more user-friendly and secure[3].wp-admin directory of a WordPress installation. This directory contains administrative files and should be protected by proper file permissions and server configurations to prevent unauthorized access[4].install.php might still be present if not updated or if the installation process was not completed. In these cases, manual intervention might be necessary to ensure its removal or proper setup to mitigate security risks.[1] WordPress Codex. (n.d.). Hardening WordPress. Retrieved from codex.wordpress.org/Hardening_WordPress
[2] WordPress Developer Handbook. (n.d.). Installation. Retrieved from developer.wordpress.org/themes/getting-started/installation/
[3] WordPress. (n.d.). About WordPress. Retrieved from wordpress.org/about/
[4] WordPress Codex. (n.d.). Changing File Permissions. Retrieved from codex.wordpress.org/Changing_File_Permissions