Models have attachment points (at least most do) that can be referenced when applying Art to them -> Origin, Chest, Head, Overhead, Weapon, Hand,Left, Hand,Right, etc.
Underneath the model is called it's "origin", you can see most Aura abilities refer to this in their Art - Target field. You can use an ability like Devotion Aura as a frame of reference to better understand how to customize your Auras. Note that not every Model will have every attachment point and not every Ability will allow you to customize it's Art the way you'd like.