Why are there giant concrete tunnels in the desert?

Fantastic!  I just discovered you recently.  Had heard a little about this, but really didn't understand.   THANK YOU. 
On the diagram at 03:30   What is the bit in the middle, looks like it is transparent to the lazor light, AND  reflective at the same time ??? 
Great explanations.  ......ps, can you do something about climate change?  in 2 ways, one, stop slow down your travel emissions.  2 more info about climate change and some * positive* news if possible?

greetings and respect from Australia April 2022.