Resources.GetBuiltinResource extension check up

Resources.GetBuiltinResource(“UISprite.???”);
check up extension Way there?

For less than I know.
C:\Program Files\Unity\Editor\Data\Resources\unity_builtin_extra
extension Rename & Copy↓
Unity_Sample\Assets\unity_builtin_extra.asset
What exists I know. but Finding of the extension do not know.

Please tell me the way to me.

use this can solve the problem.
Sprite sprite = AssetDatabase.GetBuiltinExtraResource<Sprite>("UI/Skin/UISprite.psd"); Texture2D tex = AssetDatabase.GetBuiltinExtraResource<Texture2D>("UI/Skin/UISprite.psd");

I set Unity Asset Serialization Mode (at ‘Edit/Project Settings/Editor Settings’ ) as ‘Force Text’ .
After import unity_builtin_extra.asset, it become a text file.
After searching this text file, but still, I could not figure out why the extension is “.psd” or the path is “UI/Skin”.

Maybe I miss something, Is anyone know the way?