If you find yourself needing to install an instance of macOS as a VM within VMware Fusion (which itself runs on a Mac), then you sometimes run into this issue of needing a bootable image with which to work.
There is a long history to this, but in short, macOS has had the ability to create install media buried within it for a very long time. Here are the steps to take in order to build a bootable ISO image of macOS 11.x "Big Sur".