easiest example: most components of a macbook laptop have their serial number baked in the OS (or some hardware controller, idr). So even if you can find a legit macbook part from another laptop, and you can even replace it yourself (which is also impossible but that's another story), the laptop won't boot. In order for it to boot, you need a special device which "official apple repair" people have, which can bake-in the new part's serial number.