So in Legion the leveling/questing experience is done differently than previous expansions. In previous expansions, each zone was for a certain level range. In Legion the zones scale to the player. So you can level in any zone you choose. You just go to your class hall and choose the zone you want to level in.
Here is a great article on WoWhead that explains it in more detail, and it can help you map out your optimal leveling strategy.