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: 
Runtime support is still not available using the packet manager.

You can add the preview version to your project by adding the following line to your `manifest.json` (it's located in the projects `Packages`-folder)

```
"com.unity.ui.runtime": "0.0.4-preview"

```

You may want to [check whether a newer version is available](https://docs.unity3d.com/Packages/com.unity.ui.runtime@0.0/changelog/CHANGELOG.html).

Keep in mind that the preview version is intended for previews only. The api and workflow might change with newer versions.

Once runtime support is enabled you can use the Panel Renderer as described [above](https://stackoverflow.com/a/59460866/7200161).
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.