How StreamedSceneAssetBundle differ from AssetBundle in terms of scripting?

So I look at SceneAssetBundle docs and see that all they:

  // In order to make the scene available from LoadLevel, we have to load the asset bundle.
     // The AssetBundle class also lets you force unload all assets and file storage once it is no longer needed.
     var bundle = download.assetBundle;
     
     // Load the level we have just downloaded
     Application.LoadLevel ("Level1");

in order to load entire level. I see that they do not work with assetBundle directly but load it once and then It gets into unty engine? What happens to RAM? When level objects are instantiated?

On the other hend I look at simple AssetBundle load process and it seems like no RAM shall be occupied when I simply download assetBundle bu only when I call object Load, am I correct? Or objects can be loaded otherwise than via AssetBundle.Load functions.. somehow by name even not refrencing bundle (calling AssetBundle.anyKindOfLoad)to create object instance on scene?

There is deffenetly something I do not get correctly, please tall me what?