Confused where to I write OpenCore to 1: system disk or 2: usb drive

Hi,

I'm still confused on how to use OCLP. I create the macOS Installer and write it to an external USB Drive, let's call disk drive DriveFoo. Then I proceed to the "Build and Install OpenCore" step. The machine running building OpenCore is the target machine, easy enough.

On the "Install to disk" step which disk do I write open core to:

1. The system disk
2. The external USB drive that also contains the macOS Installer?

Which one so I install OpenCore on?

Thanks!