Here are the 'pickers' inside the warehouse. They carry the 'cages' on the prongs of a forklift or 'lollop' to the relevant aisle and bay. There they load the goods onto the cages and take them to the loading bay where they are loaded onto a lorry. Or the reverse. It is an electronically synchronised process with a lot of swiping of barcodes.
It has been described as logistical choreography - down an odd aisle, back up an even one. It's a bit like dodgems but without the bumping.