Downloading and installing macOS Catalina 10.15.7 via a DMG file is a common workaround for users with older hardware or those needing to create bootable media from a Windows PC. While Apple primarily distributes macOS through the Mac App Store or System Preferences, they also provide specific DMG files for updates and manual installations. Official Apple Download Links
To clean install macOS Catalina 10.15.7 or revive a Mac that has no working operating system, you will need a 16GB or larger USB flash drive and the full installer from Method 1. How to download macOS Catalina download macos catalina 10157 dmg file install
sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume