That depends on the architecture. E.g., Intel uses a wired-OR circuit and the cores race to determine who booted first, then that core becomes the boot core and executes the first instruction from boot ROM.
I'm assuming a simplistic CPU architecture aimed towards beginners, which is generally what a tutorial is aimed at. From there you can learn about all the nitty gritty details that you need to get actual chip to work.
I think the point is that on a standard x86 PC the cores don't all start running from reset; one boots up and the others are halted, waiting for the OS to start them (usually via the APIC.)
