I was fascinated by this process! Don’t know why I never really considered it before.
The concrete is poured and smoothed over. Concrete stamps are laid on top of the wet concrete like a jigsaw puzzle, and pressed down to create the cobblestone impression.
The last piece of flexible stamp is carefully lifted off…
…and laid down as the next piece at the other end. The order of stamp placement keeps the cobblestone pattern looking random.