Built-in extra asset

private const string kStandardSpritePath = "UI/Skin/UISprite.psd";
private const string kBackgroundSpriteResourcePath = "UI/Skin/Background.psd";
private const string kInputFieldBackgroundPath = "UI/Skin/InputFieldBackground.psd";
private const string kKnobPath = "UI/Skin/Knob.psd";
private const string kCheckmarkPath = "UI/Skin/Checkmark.psd";

Image image = buttonRoot.AddComponent<Image>();
image.sprite = AssetDatabase.GetBuiltinExtraResource<Sprite>(kStandardSpritePath);

https://forum.unity.com/threads/accessing-ui-build-in-sprites-in-c.305075/