Use the official Apple link to download the InstallMacOS.dmg file [3].
A involves erasing your hard drive entirely and installing the operating system from scratch. This is vastly superior to an "upgrade" or "reinstall" because it:
The installation process may take 30–60 minutes and will restart the Mac multiple times. Use the official Apple link to download the InstallMacOS
If you are looking to run macOS Catalina in a virtual machine (e.g., VirtualBox or VMware) rather than on bare metal, you may need a pre-created, full installer DMG.
Download macOS 10.15 Catalina DMG File for Clean Installation (2026 Guide) If you are looking to run macOS Catalina
Run the .pkg file inside to install the "Install macOS Catalina.app" into your /Applications folder.
Never attempt a macOS installation on battery power. Crucial Step
Crucial Step! Use Time Machine to back up all important files to an external drive. A clean install erases everything.
I can provide the specific boot key commands or virtual machine settings tailored to your hardware.
Enter your administrator password and type Y when prompted. Step 3: Performing the Clean Installation