Convert macOS installer to ISO

As of macOS Mojave, Apple no longer provides a DMG file to install the macOS, it only provides an .app file that needs to be converted to ISO if you want to use it in a virtualization software.

Step-by-step guide

Here are the steps needed to convert the .app file to ISO

• Download macOS, can be found here

• Create a virtual disk for the installation media:

hdiutil create -o ./BigSur -size 13000m -layout SPUD -fs HFS+J

The following parameters are customizable, depending on the location and the size of the installer:

• ./BigSur - location where you want to create the virtual disk
• 13000m - size in megabytes; size of the Big Sur installer is 12.24 GB, or translated to megabytes, it's 12240 megabytes; 13000m virtual disk size should allow for any additional headspace needed
  Results:

nex-macpro2019:MacOSApp nex$ pwd
/Volumes/BigDrive/MacOSApp
nex-macpro2019:MacOSApp nex$ hdiutil create -o ./BigSur -size 13000m -layout SPUD -fs HFS+J
created: /Volumes/BigDrive/MacOSApp/BigSur.dmg
nex-macpro2019:MacOSApp nex$ ls -alh
total 37729792
drwxr-xr-x   8 nex  staff   256B 31 Dec 15:55 .
drwxrwxr-x  13 nex  staff   416B 30 Dec 20:11 ..
-rw-r--r--@  1 nex  staff    13G 31 Dec 16:06 BigSur.dmg

Please note that all of the commands are ran under the folder/directory /Volumes/BigDrive/MacOSApp

• Mount the disk:

hdiutil attach ./BigSur.dmg -noverify -mountpoint ./BigSur

Results:

nex-macpro2019:MacOSApp nex$ hdiutil attach ./BigSur.dmg -noverify -mountpoint /Volumes/BigSur
/dev/disk5              Apple_partition_scheme
/dev/disk5s1            Apple_partition_map
/dev/disk5s2            Apple_HFS                       /Volumes/BigSur

• Write the installer to the mount point:

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

This process will take some time, depending on the speed of the disk you're writing to.
Results:

nex-macpro2019:MacOSApp nex$ sudo ./Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/BigSur
Enter PIN for 'Certificate For PIV Authentication (Yubico PIV Authentication)':
Ready to start.
To continue we need to erase the volume at /Volumes/BigSur.
If you wish to continue type (Y) then press return: y
Erasing disk: 0%... 10%... 20%... 30%... 100%
Copying to disk: 0%... 10%... 20%... 30%... 40%... 50%... 60%... 70%... 80%... 90%... 100%
Making disk bootable...
Install media now available at "/Volumes/Install macOS Big Sur"

At the end of this process, you will have an ISO install image at the chosen destination folder.