Apple has steps to create a bootable USB (linked below), but not an ISO file.
https://support.apple.com/en-us/HT201372

There are also several methods to create an ISO, such as colt2’s method linked below.
https://forums.macrumors.com/threads/el-capitan-bootable-dvd.1923931/post-22048516

The steps in this guide are what I found to be the most straight forward compared to other guides. These are performed on an OS X system to create a bootable ISO which can then be imaged to USB or used directly for installing on virtual machines.

Note – the ISO created from this method does not work when burned to DVD.  The resulting disk is bootable, but when trying to install the operating system you will receive the error:
The Install OS X El Capitan application can’t be used from this disk. Copy this application to a writable Mac OS Extended formatted disk and reopen it to continue installation.

The steps presented below can be used for other versions of OS X by increasing the size of the image as necessary and adjusting the name of the files appropriately (from El Capitan).

1. Download El Capitan installer from Apple
   http://updates-http.cdn-apple.com/2019/cert/061-41424-20191024-218af9ec-cf50-4516-9011-228c78eda3d2/InstallMacOSX.dmg
2. Install it
3. Create a blank image file in the temp folder, overwriting any existing file that may exist

   hdiutil create -ov /tmp/ElCapitan -size 7316m -layout SPUD -fs HFS+J

4. Mount the blank image to a temporary mount point

   hdiutil attach /tmp/ElCapitan.dmg -noverify -nobrowse -mountpoint /Volumes/my_bootdisk

5. Write the installation files to the image (this step requires administrator privileges)

   sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/my_bootdisk --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app

6. Unmount the new image

   hdiutil detach /Volumes/Install\ OS\ X\ El\ Capitan

7. Convert the new image to an ISO file

   hdiutil convert /tmp/ElCapitan.dmg -format UDTO -o /tmp/ElCapitan

8. Move the file from the tmp folder to the Desktop of the logged in user and also change extension to ISO

   mv /tmp/ElCapitan.cdr ~/Desktop/ElCapitan.iso

9. Delete the original image file

   rm /tmp/ElCapitan.dmg

The resulting ISO file is now ready to use.

I had an OMV system running 4.1.35-1 (Arrakis). Initially I tried using the steps from OMV’s ryecoaaron: https://forum.openmediavault.org/index.php?thread/27909-eta-for-omv-5-0-2020-2021-2022/&postID=219830#post219830
(This included having to use a Python fix for the weakref.py error: https://jc-lan.org/2020/04/26/openmediavault-weakref-python-error-when-updating)

However my progress came to a halt at this step:
omv-salt deploy run nginx

When I received this error
-bash: omv-salt: command not found

Below are the alternative steps I used to complete the upgrade. At a high level we first upgrade the Debian OS, then go back and upgrade/repair OMV package. It is based on this guide: https://blog.sakuragawa.moe/upgrade-omv-4-to-5-a-painful-path/

1. sed -i 's/arrakis/usul/g' /etc/apt/sources.list.d/openmediavault.list

2. sed -i 's/stretch/buster/g' /etc/apt/sources.list

3. apt update

4. On the next two steps, if prompted then choose: Don’t touch keymap, install all package maintainer’s configuration files (several of these) and restart services during package upgrades without asking

5. apt -y --auto-remove --allow-downgrades --no-install-recommends upgrade

   apt -y --auto-remove --allow-downgrades --no-install-recommends dist-upgrade

6. Now reboot

7. apt -y purge openmediavault

8. apt install -y --auto-remove --allow-downgrades openmediavault

9. apt-get install usrmerge

10. omv-confdbadm migrate conf 5.0.0

11. omv-confdbadm populate

12. sudo apt-get -y purge php7.0*

13. omv-firstaid

14. Choose option 9 – Clear web control panel cache
15. Try to access OMV web interface
16. Perform a final reboot to fix any broken services
17. NOTE: if you had notifications enabled previously, log in to the web interface and disable them under the General Settings. Apply changes, then immediately revert back to Enabled and apply changes again.
18. OPTIONAL: if you had a static IP and DNS server configured you will need to reconfigure your name server. In the web interface remove it, save then add it back. OR you can do the following:
    nano /etc/resolv.conf
    Add a line with the IP of your DNS: nameserver x.x.x.x
19. OPTIONAL: reinstall OMV-extras (https://omv-extras.org)