Now for the locking mechanism.
The first photo shows a side view of the truck chassis - note the two holes in the central "spine" of the chassis, one at the rear and one towards the front. These accommodate the locking pegs which attach the container to the truck. The trailer chassis follows the same design.
In this underside view of the container the central "spine" of the truck/trailer chassis slides down the central channel on the underneath of the container. Here, the support legs are pulled clear of the trailer body and the locking pins which are part of these are retracted, leaving the central channel unobstructed for the container to slide onto the chassis.
This second picture of the underside of the container shows it with the support legs pushed in as they would be to lock the container on to the chassis. The locking pins are clearly visible in the central channel and would be pushed into the holes on the central spine of the chassis, locking the container in place.