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