Also, the DMG file is the disk image used to create the bootable USB. So the user might actually need instructions on making a bootable USB from the DMG or from the Install app.
Also, check if the user is on a Mac that supports Catalina. Macs older than 2015-2016 might not support Catalina. They need to verify their Mac's compatibility. Also, the DMG file is the disk image
Another angle: the user might not have a macOS with the App Store installed yet, or they want a direct link for a clean install. But for a clean install, you typically use Disk Utility and a USB drive. So maybe they're trying to bypass the App Store download. Macs older than 2015-2016 might not support Catalina
I should outline the steps clearly, mention the need for the App Store download, provide the link to Apple's support page for older macOS versions, and explain the steps to create a bootable USB. Also, warn against third-party downloads to avoid malware. If they can't download from the App Store, suggest alternative methods, but only the safe ones. But for a clean install, you typically use