Once the installer is in your folder, you can use the Terminal to convert it.
Here’s the immediate reality check: Unlike Windows or Linux distributions, Apple distributes its operating system exclusively through the Mac App Store as a .app bundle (for older versions) or a .pkg installer. However, converting that official installer into a 64-bit ISO for virtualization is entirely possible and legal, provided you own Apple hardware.
Downloading macOS from third-party sites is generally risky, though common. Using softwareupdate (Method 1) is entirely legal and safe, as it pulls directly from Apple servers. What is the difference between dmg and iso?