Correct. Whatever one is installed closest to the CPU (top most PCIe slot). That’s GPU 0. Unless you tell the OS or application otherwise, that’s what it uses by default for actual GPU processing.
Edit: Just FYI, I do run multiple GPUs. I have a 3070Ti as my primary, and run an old Quadro P620 (GT1030 equivalent) as a secondary to run my 4 touch screens for my Air Manager cockpit.