If you ever lurked in Godot source code, and tried to follow the flow of the logic, you most likely noticed that most code related to scene, formats, etc. always ends up in a giant "server" class. These really large classes, which Godot calls "severs", generally abstract some implementation or architecture.
Godot has many built-in types. Built-in types are used for non-pointer API arguments, where you need to pass around information fast and you don't really care much about keeping a reference. One of the early built-in types in Godot is Image, which is like a Vector, but with a little more information related to image data (such as width, height, format and whether or not it has mipmaps).