As I understand it, you need an ISO image to create a virtual machine. Is that not correct?Why do you need an ISO image? Fusion can create a Big Sur client using the standard Apple supplied "Install macOS Big Sur".
If you really need an ISO then make one yourself. See, for example, https://osxdaily.com/2020/12/14/how-create-macos-big-sur-iso/
Fusion does the required magic.As I understand it, you need an ISO image to create a virtual machine. Is that not correct?