Install OS X El Capitan on an external HDD from macOS Catalina

To perhaps explain in more detail

I have a Mac Pro early 2008 machine and a Mac Pro 2019 machine. I've forwarded the early 2008 machine drives (unaware than getting the installer is going to be so challenging). I am unable to open the El Capitan package to get the installer on my 2019 machine.

I just need a bootable version of El Capitan, or to install on an external hard drive so I can reboot my early 2008 machine.

Thanks

Hi,

"El Capitan downloads as a disk image. On a Mac that is compatible with El Capitan"

On a Mac that is compatible with El Capitan. I am trying to create a bootable El Capitan installer from a macOS Catalina (and hence non El Capitan compatible) machine.