After installing Catalina on a machine I realized some programs I want to run on this machine are still 32-bit. So I went to downgrade from Catalina to High Sierra and since there's no way to re-run the High Sierra installer once you install Catalina, knowing that I had to reformat my drive I rebooted into Disk Utilities. (in my case I happened to be booting into a High Sierra installer that is on a USB bootable stick).
Unfortunately I made the mistake of deleting my Volume (not just reformatting the drive), which is on the left in Disk Utility, and now I can't get it back. I don't mean "get it back" is in recover the data (there is no data to recover-- the disk was empty), I mean I can't even see the drive mount at all in the toolbar of Disk Utility.
I now have both a High Sierra and Mojave bootable disks, and when I boot into the High Sierra disk I don't see the 500 GB internal disk "Volume" in the left of the toolbar.
How can I reformat the drive to reformat the 500 GB internal disk (this is on a Mac Mini 2014)?