On OSX Catalina - how to create a bootable disk of Mojave

on a newer Mac running OS X Catalina I want to create a bootable Disk of OS X Mojave for an older iMac that is running Mojave. When I click on the macOSUPDCombo10.14.6.pkg file that I downloaded to create this install media file in the Applications folder on the newer Mac I get this message: "This software is not supported on your system." Is there a way to create this media file on my Mac using Catalina so I can create this bootable disk of OS X Mojave on a USB stick (a downgraded installation disk if you like) . Any idea how to work around this? Thx