The base is the squarish thing. It has a metal lid. The metal lid has two holes in it about 1″ in diameter, and the big hole for the lowest pot to sit in. There are caps which go into those holes, reducing them to smaller holes. The caps open up so that you can thread the pump plug through them then close them again and reduce the airflow/bugs/gerbils whatever.
There is a diffuser that goes at the top end of the PVC.
The threaded male PVC fitting at the top takes a plastic washer, then a bowl with holes in it, then you put the mushroom bubbler on the threads to hold everything in place. The mushroom bubbler makes sure that the water doesn’t all go to one side if your tower isn’t perfectly level (I think).