In short, to make a boot disk for MacOS:

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

I had no idea Apple was including a shell tool as part of the OS distribution, specifically for making bootable disks. And apparently it's been around for quite some time. I always squirrel away all the OS releases for testing, so this is super helpful for me.