Being included Create New → Resource → Create New Resource window
Can be created specific to scene/node without explicit presence in the Editor’s file system (Like how StyleBox can exist specific to a Theme resource)
Is possible for custom user-made Resource, and how I can do it in GDscript and C#. Thank you in advance!
I’m not sure what you mean. If you give a script that extends Resource a class_name it will show up in the Create new Resource popup and you can @export it by using that class_name as the variable type. More info here Resources — Godot Engine (stable) documentation in English