I'm using Vuforia for developing AR apps for Android using Unity as a editor. But this uses a target as in images/3D objects etc. But what I want is an app that recognise the environment it is in, like this app Augment App for Android.
In this app we can place our 3D objects on any terrain and scale or rotate it. I want to build this kind of app but the use case is completely different.
I have tried Kudan, but its a paid SDK.
Markerless tracking is a research field in progress. There are a couple way this is done, but nothing really works well. Even the app you linked uses markers to display models. For this purpose Natural Features (like in Vuforia) are used to distinguish between the environment.
In the screenshots, where it does not use markers it does not use tracking at all. It uses the camera feed and then overlays this with a virtual object, which is previosly uploaded and subtracts the movement of the camera to the location of the object to give the illusion of standing in place.