How "get All Actors Of class" node generates the drop down class list?

as the question described .
any clue?

this question actually related to this issue:
How do I get a list of all blueprint classes(in blueprint or in c++)? - Programming & Scripting - Unreal Engine Forums

In blueprints from the “get all actors of class” select your class and from the array node call an “for each” loop then use the array element node to find what you need.

i know how to use it.
i was looking for what actually happens in background of that node.

void UGameplayStatics::GetAllActorsOfClass(UObject* WorldContextObject, TSubclassOf ActorClass, TArray& OutActors)

In the C++ it’s just a TSubclassOf object. Blueprint will automatically create a dropdown for TSubclassOf and list all the actors you can use in blueprint. If you want to know how to make a drop down for just anything using Slate, or how to get a list of all Actors, I don’t know off the top of my head.

i found that out,
actually i need to know how the drop down menu filled with class list.
it happens through:
static void GetClassList(…)

but that function deals with FClassViewerNode.
and there is no clue about getting the list.
just nothing