Question: I installed the new Unity UI Builder. Everything works just fine...

But how do I add the uxml to my scene or how do I attache it to camera? 

I tried to drag and drop the uxml to camera and I searched for a component, but I couldn't find anything... Is there any official documentation?

Answer: 
You must use an gameObject with PanelRenderer.cs attached to it:
[![Panel renderer in order to use uxml in runtime](https://i.stack.imgur.com/bgp8C.png)](https://i.stack.imgur.com/bgp8C.png)

For now, I only know you have to set UXML file and USS file. In play mode, by magic (I still don't understand how unity does it), it will render.

In the Github project you can see how things are for now. [UIElementsUniteCPH2019RuntimeDemo](https://github.com/Unity-Technologies/UIElementsUniteCPH2019RuntimeDemo) 

**EDIT:**
The project linked above uses UI Builder 0.8.
In Unity 2021.2, when you add a UI Document to the hierarchy, a PanelSettings asset is automatically created and assigned to the that UI Document.

Simply add your uxml file to the UI Document -- there is no Panel Renderer component anymore.