The video tutorial of this topic can be found here.
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The 3D model bubbles on the left hand side helps you keep track of the 3D models on the scene.
When you select a 3D model, the corresponding bubble will increase in size.

You can also select a 3D model by tapping on it's corresponding bubble. Once doing so, that 3D model's toolbar will appear so that you can change it as you see fit without having to select the 3D model directly from the scene. 

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