They don’t put that stuff in the user manual because they assume that people already know “why” — the manual is only to direct the new user to the “how” that is particular to their piece of hardware.
I suggest you pick up an introductory book to digital videography. Even a beginners book to standard photography can help. Again, I don’t want to sound condescending, I just really feel it would be helpful for you to understand the “why” and familiarize yourself with some basic terminology.
For example, an experienced cameraman would know when someone says “aperture” it means the same thing as “iris”. They would know that it controls the amount of light allowed through the lens, and how adjusting it effects not only your exposure, but also your depth of field.
At this point in your experience, you really, REALLY want to get knowledgeable about all this stuff. It’s what separates the professionals from the amateurs.