New pool owner, pool doesn't use chlorine or salt, confused

I am a new pool owner and I know NOTHING about pools other than they are wet. I have a few questions.

The pool appears to be made of some sort of concrete like material. From googling I think that is called Gunite?

The pool is in-ground. It is 17x25ft. The shallow end is 4ft deep and extends the first 14ft of the pool. Then it slopes down to 8.5ft. Is it possible to estimate the number of gallons this pool holds?

There is a sand filter, a CO2 tank, and a translucent tube with two metal bars inside hooked up to electricity. Can someone explain this system to me? I was told it is neither salt or chlorine.

Thanks