Where do you download the Unity headers for C dlls?

The [manual][1] states:> See IUnityInterface.h for the correct signatures. IUnityInterfaces is provided to the plugin to access> further Unity APIs. Well great, where are they? There is no mention anywhere of the download location of these headers... Does anyone know where they are hosted? [1]: https://docs.unity3d.com/Manual/NativePluginInterface.html