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
Boot from USB: Restart your Mac while holding the Option (Alt) key and select the installer volume to begin the process. 4. Installation for Unsupported Macs download macos catalina 10157 dmg file install
To perform a full installation, you must first obtain the installer app and then convert it or use it to create bootable media. 1. How to Download the Official Full Installer Downloading and installing macOS Catalina 10
Upgrading: Simply open the "Install macOS Catalina" app from your Applications folder and follow the on-screen prompts. How to Download the Official Full Installer Upgrading: