Can someone explain this wiring configuration to me?
They are buses

Hang onto your hat.  This is one of the few times wire color codes actually mean something.   In this case (not often repeated): 


Black is always-hot
Red is switched-hot
White is neutral (well, that actually is fairly common)


All those wires have simply been bussed to all locations.  Remember


Supply needs always-hot and neutral 
Dumb switches need always-hot and switched-hot (but neutral must be present) 
Smart switches need all 3
Lamps need switched-hot and neutral
Split receptacles need all 3 


Here's a drawing I did of a bussed arrangement for sharing motion sensors.  It's exactly the same deal. 



Nevermind my error with the ground wires.  You get the idea.