Create macOS Big Sur ISO Image File

First download macOS Big Sur From Apple with App Store. Quit installator with ⌘+q, then create new empty volume:

sudo hdiutil create -o /tmp/BigSur -size 16384m -volname BigSur -layout SPUD -fs HFS+J

Mount new volume:

sudo hdiutil attach /tmp/BigSur.dmg -noverify -mountpoint /Volumes/BigSur

Create bootable dmg and copy to new volume:

sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/BigSur --nointeraction

Eject installator:

hdiutil eject -force /Volumes/Install\ macOS\ Big\ Sur

Convert dmg to cdr file:

hdiutil convert /tmp/BigSur.dmg -format UDTO -o ~/Downloads/BigSur

Rename cdr to iso:

mv -v ~/Downloads/BigSur.cdr ~/Downloads/BigSur.iso

Delete original created dmg file:

sudo rm -fv /tmp/BigSur.dmg