how to create bootable USB disk for El Capitan from latest macOS Catalina

Hi There,

while trying to reset my old iMac (early 2009), I tried to "Reinstall macOS" from the recovery mode, and got the "Mac OS X could not be installed on the computer", without giving me any specific reason or details. Apple suggests I create a bootable USB, which can be used to install, but I am unable to as I am hitting deadends. For example, the instructions here do not cover my need

https://support.apple.com/en-ca/HT201372

when I download disk image for the "Install Max OS X" all (that creates bootable installer) for OSX El Capitan or Yosemite, I can not run it on my MacBook with Catalina (don't know why.. backward incompatibility perhaps?).. and my iMac can not run anything newer than El Capitan.. What can I do?

I'm avoiding having to create a new partition on my latest MacBook for El Capitan, just to create a bootable image disk.. it would be ideal if I can download it from somewhere..

Thanks much for your help.