Okay, I’ll try to go through it:
1 - Basically, yes. The entire engine is written in C++ as are many other applications. It’s one of the industry-standard programming languages. And yes, components are classes themselves. As a programmer, the most commonly-used method is “Object-Oriented Programming” (Google it if you don’t know what it is, basically, search for any terms I use here if you don’t know what they mean, it takes way too long to explain them all). OOP means you “inherit” from one class to add additional functionality for a new more specific class. So in UE4, UObject is the highest class, followed by AActor which “inherits” from UObject (making UObject the “parent” of the “child” AActor). Then SceneComponent is a child of AActor, and ActorComponent a child of UObject.
2/3 - The Root Component is by default an empty SceneComponent and is responsible for giving the Actor its transform properties. It’s not a class but a variable within AActor, which by default automatically creates an empty SceneComponent. The Root Component is responsible for the transform of the Actor as a whole. So in the Static Mesh Actor, the Static Mesh Component is the Root Component and handles the transform for the Actor. The same applies to the Point Light.
4 - They’re not technically two languages. It’s still all C++. However, Unreal Engine has an “API” (Application Programming Interface). This is basically nothing more than a collection of classes (UWorld, UObject, AActor, etc.), types (FString, FVector, etc.), macros (TEXT, GENERATED_BODY, UE_LOG, etc.), and libraries of functions, that are implemented in the engine and can be used by programmers to build a game without having to write really low-level code. This way you don’t have to go very deep into the core of C++ code to create games, which is the whole point of using a game engine.
I understand it can be difficult, it took me many years before understanding code as well, let alone C++. The irony is, is that this makes it simpler than using plain C++. In fact, it’s Unreal’s API that allowed me to better understand C++ in general, even though I can’t hope to build anything with C++ alone. Remember, most of this isn’t specific to UE4, but rather C++ itself. “Classes” are the most basic thing in programming languages like C# and C++. I suggest picking up a book, either about general C++, or about C++ in UE4 if you want to work with UE4 specifically. Believe me, those are invaluable to have. I also recommend reading about Computer Science if you’re really interested in programming in general. That really deals with understanding computers, which is important as C++ isn’t called a “language” without a reason.