I’m trying to put the camera viewport as a texture on this plane placed in front of the iPhone. Can you help me with this? I don’t know where to put scripts and how to it up because I’m new to this. Your help would be greatly appreciated. 
This is a little tricky as you will need a second camera that is rendering everything but the texture that you are applying it to. The second camera would be rendering to a texture as a render target.
This can be done via render layers where the texture for the phone display is rendered by your main game camera but not by the camera for the phone.
This does mean you are rendering everything twice though and that will hurt performance.
This project can help where the screenshot is rendering everything but the UI )(https://developer.playcanvas.com/en/tutorials/capturing-a-screenshot/)
The difference here would be that you don’t need to apply the render target pixel data to the canvas, you will just apply it to a material that is used in the scene.