Back in Chapter 2, The Building Blocks of Programming, we discussed how a script is transformed into a component when it's added to a GameObject in Unity. Think of this in terms of the OOP principle of composition—GameObjects are the parent containers, and they can be made up of multiple components. This might sound contradictory to the idea of one C# class per script, but, in truth, that's more of a guideline for better readability than an actual requirement. Classes can be nested inside one another—it just gets messy fast. However, having multiple script components attached to a single GameObject can be very useful, especially when dealing with manager classes or behaviors.
Always try to boil down objects to their most basic elements, then use composition to build bigger, more complex objects out of those smaller classes. It's easier to modify a GameObject made out of small, interchangeable components than one large,...