Apple does not provide a direct DMG download link on its main website; instead, it offers the installer through the App Store or Terminal. 1. The Official App Store Method (Recommended)
This is the safest way to get the installer directly from Apple's servers. Download Macos 10.15 Catalina Dmg File
: These third-party utilities simplify the process by providing direct download options for various macOS versions, including Catalina, and offer built-in features to create bootable USB drives. System Requirements for macOS Catalina Apple does not provide a direct DMG download