Once you have the safe DMG, here’s what you can do:

Apple’s support article (ID: HT211683) usually contains a direct App Store link. When clicked, it forces the App Store to launch the download. This is the only 100% legal, virus-free method.

command in Terminal after downloading the installer app from Method 1 or 3. Apple Support Terminal commands

While macOS updates are technically free, the internet is rife with "free download" traps for legacy software.