modify the URP terrain shader?

Im using the new terrain with URP and i like to know how can i customize the terrain shader ?
the terrain material itself can be modified but i cant find where to modify the tree or the grass material (or shader).

Unity doesn’t have a way to edit the shader because terrains work differently. In the dropdown menu where it says Raise or Lower Terrain in the inspector, select Paint Textures instead and click on the plus sign and click on add terrain layers. You should not see anything. This is because you don’t have any terrain layers. Import the terrain samples made by Unity Technologies from the Asset Store which is a collection of some good terrain layers. Now add a terrain layer and you should see a bunch of options. You can also create and edit terrain layer assets.