hdiutil convert ~/Desktop/BigSur.dmg -format UDTO -o ~/Desktop/BigSur.iso

Apple does not provide direct ISO downloads for macOS. You must download the official installer from Apple (as a .app bundle) and then manually convert it to an ISO. The ISO is typically used for creating bootable USB drives for Hackintoshes or virtual machines (VMware, VirtualBox, etc.) on non-Apple hardware.

(Note: volume name may change to Install macOS Big Sur after step 4.)

Here’s a step-by-step guide to legally download a macOS Big Sur ISO file.