Know little about spey but bought one. Can learn how to cast with videos I bought. But how do you FISH with a spey rod?

Seems you cast a long distance. So you are in a fast moving river:
Where do you cast to....
Upstream?
Downstream?
Doesn't all the moving water wreak havoc with all that long line in the water?
How do you know when you have a strike...end of line is so far away in fast water...
How do you set the hook?

Just curious....might go to a river next week and no nothing....and it shows....