Trying to get understanding... within unreal....#include "GameFramework/Actor.h"

Not directly, but if you look at the contents of the “YourProject/Source/YourProject/YourProject.Build.cs” file, that is where the dependencies of your primary game module are listed. The documentation includes the complete module list, but not all of them are useful for every game, which is why they’re not included by default.

The module system used in Unreal C++ isn’t actually part of the C++ language or Visual Studio, but of Unreal itself. The UnrealBuildTool program is what reads those .Build.cs files and sets up the project so that it can be compiled, and it’s automatically called every time you build your game through Visual Studio or start a hot reload from the editor.