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).
1. Even though the UI Builder offers a preview of the menu over the actual game scene, there is no way to use that menu in a game using Unity's functionality or packages. To do that we need to use the code that comes with the demo by Damian Campeanu from Unite Copenhagen 2019 that you can find here: <https://github.com/Unity-Technologies/UIElementsUniteCPH2019RuntimeDemo>.

The demo has following structure.
[Structure of the project](https://i.stack.imgur.com/pqDrL.png)

2. The files that we are interested in are located in Assets/UIRuntime folder. Simply copy this folder into Assets folder of your project.
3. After copying the the folder,create empty game object, and add two components in the inspector. Panel Scaler and Panel Renderer.
[Game menu object with panel scaler and panel renderer components](https://i.stack.imgur.com/hT2Ih.png)
4. Panel Scaller is responsible for scalling your menu for the display. Choose "Constant Physical Size" and 96 for both Reference and Fallback DPI. Those are the values from the original demo and 96 DPI is a typical pixel density for most computer screens (it is different on mobiles and high density screens).
5. Panel Renderer is the place where you will set your UXML and USS (Unity Style Sheet) files (Uxml and Unity Style Sheet fields. Leave the last two of fields empty, and tick the "Enable Live Updates" checkbox. This will enable you to show the updates in the game viewport as you make them.

I wanted to comment under existing answer by Pablo but I'm new so I can't.