Booting from external, bootable USB-C thumb drive

I have created a bootable USB-C thumb drive using the Apple Recommended process in "How to create an external USB Bootable Drive" (https://support.apple.com/en-us/HT201372) and used it to install macOS Catalina 10.15.7 in a newly created volume on my 2018 Mac mini which came with Big Sur pre-installed. It runs Catalina just fine, thus you can install an earlier version of macOS than what came pre-installed on the Mac. After discovering that Catalina will not run 32-bit programs, and I have some 32-bit programs I need to run on my new Mac mini (64-bit versions are not available), I created another external USB-C Bootable Drive to install macOS High Sierra 10.13.6 on another newly created volume. However, this time, my mini will not let me use the USB-C thumb drive as a startup drive using the Option key held-at-startup process. It recognizes and lists the USB-C thumb drive as a startup drive option, but when selected for startup, it displays the international prohibited symbol and the mini shuts down. I have re-created the High Sierra install thumb drive in case the original creation was corrupted or flawed with the same result. The Startup Security Utility settings have been changed to allow booting from an external removable media and the security setting set to none to prevent its interference in the install process. Apple's support web page lists the 2018 Mac mini (2012 and later) as compatible with macOS High Sierra. So why can I not use the USB-C drive to install macOS High Sierra on my mini? Also, is High Sierra compatible with the APFS format of my newly created volume, or does it need to be reformatted to MacOS Extended (Journaled) - if/once I get my mini to use the USB-C thumb drive as a startup drive to install High Sierra?