Your Mac must meet these minimum hardware specifications to run Catalina: At least 4GB of RAM (8GB recommended).
If you are already on a Mac, open Terminal and run the following command to download the full installer: softwareupdate --fetch-full-installer --full-installer-version 10.15.7 mac os catalina 1015 7 dmg download free
Apple typically provides macOS Catalina through the Mac App Store or via terminal commands rather than a direct public DMG file for the full installer. Your Mac must meet these minimum hardware specifications