Hi, this Portal Effect is not a AR engine functionality, it's an effect that can be achieved just with Unity Shaders and it works with any of the AR engines out there.
It's done enclosing your entire scene in a hollowed box, that box must use a Matte/Mask Shader to "hide" everything it's behind it. That shader is quite simple to do:
You can download the one made by Qualcomm(Vuforia): https://github.com/aholla/Vuforia-Test-Unity/blob/master/Assets/Qualcomm%20Augmented%20Reality/Shaders/DepthMask.shader
I use this shader to do the deep hole on the number 10 in the center of this scene: