I'm looking for a way to interact with an existing GameObject (as andy in HelloAR).
I saw this post :
Detecting if an tap event with ARCore hits an already added 3d object
which partially answers the question, but I'm using the latest release ARCore Developer Preview 2 in Unity in C# not Java.
I'm learning, so I'm not looking for code but more in tutorial or explanation about the User Interact Concept in ARCORE
Touching 3D objects is nothing you solve using ARCore. It's in the responsibility of Unity. Everything you need is covered in this Unity tutorial https://unity3d.com/de/learn/tutorials/s/roll-ball-tutorial. It's just touching a 3D object in a Unity scene.