Re: Booting to Usb upon start up to GRUB?
On the page I linked to, I interpreted it to mean that pressing the C key during startup would bring up a boot menu, where one of the choices would be a USB drive (assuming you have a bootable USB drive plugged in). Or, pressing the Option key would bring up the "Startup Manager" (where you could set the boot order?). This would be handled by the BIOS or EFI BootROM, before GRUB is even started.
Maybe I misunderstood what you are asking? It sounds to me like you wiped your MacOS to install Linux (Fedora?) & now you want to go (back) to OS X (instead of, or in addition to, Linux?). If so, a Mac forum might be more helpful (for that part).