But what will be the procedure when the computer is booted for the very first time(out of the factory, anew)?
On modern PC mainboards with SPI flash (8 pins), there's often a set of pins (also 8) in close proximity to the flash chip - attach an external flasher to that, and you can write the flash while the PC is turned off.
The alternative would be to write the flash before it's put on the board, but which method is chosen is an implementation detail.
That is to say, the reset vector is a pointer or address where the CPU should always begin as soon as it is able to execute instructions.
In other words, people who have lots of experience in this stuff gloss over the fact that "reset vector" doesn't mean reset at all, it actually means "whenever it's turned on, it jumps to this address and starts executing instructions".