I've only done custom cables once and I followed BitWit's guide on using paracord to sleeve them yourself. I bought a spool of wire and all the pins and crimper and such and made my own cables (time consuming and tedious, but worth the experience to do once). I had to use a multimeter to pin out the cables that came with my power supply to make sure I had the same pin out, the wires were all over the place from one end to the other.

Question: are prepurchased Custom cables the same way? From all the pictures I've seen, they just match up the holes in the connectors, there's no way it matches an actual pinout. Or are those pics just generic stock photos that don't match the actual product?