Where does pool backwash go?

It goes out the backwash line.

This is usually either plumbed into the line, and either goes into the sewer, or out on the ground.

Some setups have a hose that you unroll, and attach to a port on the valve.

I found my discharge by putting the handle on "backwash", turning the pump on, and looking around! Mine happened to go out into the back ditch. Took all of about 10 seconds to find it.