Before downloading and installing macOS Catalina 10.15.7, make sure your Mac meets the system requirements:
Version 10.15.7 was the final major update for Catalina. It includes critical security fixes and addresses a specific issue where macOS wouldn’t automatically connect to Wi-Fi networks. It is the most stable "end-of-life" version for this operating system. Creating a Bootable USB download macos catalina 10.15.7 full installer
Then run the installer. After installation, reconnect Wi-Fi and run sudo sntp -sS time.apple.com to re-sync. Before downloading and installing macOS Catalina 10
Apple maintains a hidden directory of full macOS installers, even for older versions. The safest, most legitimate method uses the softwareupdate command-line tool or a direct link from Apple’s content caching servers. even for older versions. The safest