MickeyDee.... The "cages" are just a box made from eggcrate that surround the intakes for the closed loop (one cage for two intakes). They are located in the rear of the tank behind, but not under, the rockwork. They are easily removable for cleaning. The jets on the closed loop are a different story though... some are very easy to get to... while others have to have a few rocks removed to get to them. I don't anticipate ever having to remove them for cleaning though; but, if I had to, it wouldn't be too hard.
There is no sand under the rocks... only in the visible areas. When I set up the tank, the first thing that I did was to put in the closed loop piping, then the rocks to completely cover the pipes, and then the sand only in the open areas (and only about 1 inch or less deep). The closed piping is hidden by rocks.
I'm glad you couldn't see any plumbing or overflows in the photos... that's the idea. There are two overflows... one in each of the rear corners that measure 12 X 12 inches each. They are mostly obscured by rockwork without impeding flow. I also have no lights over this area which places this area in a kind of shadow that draws your eye back towards the middle of the tank and away from the overflows.