How can I make a bootable USB copy of Mojave downloaded from Apple when the file is an app and not a disk image?

When I try to open the Mojave install app on my computer with Big Sur, it says that the operating system is too old and so it will not let me open it in order to install it on the external usb drive.

Additionally, the alternative method - restoring Mojave onto the external drive is not possible because the Mojave install file is an app and not a disk image.

How can I proceed to create a bootable copy of Mojave on my external USB drive considering I am on a computer with Big Sur and the Mojave download is an app file which will not open and it is not a DMG file which I would otherwise be able to restore onto the external USB?

Follow the createinstallmedia directions here: How to create a bootable installer for macOS

For the sake of completeness, here’s the command you would run in Terminal to create the install media for Mojave (it’s slightly different for each OS version):

sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume