Another good book is The Science of the Singing Voice by Johan Sundberg, but it's out of print. You can still try ordering it used. If these books don't confuse you enough, try Great Singers on Great Singing by Jerome Hines. It'll give you some perspective on the varieties of experience singers have. For work on the technique of breathing alone, try The Art of Breathing by Nancy Zi.
Breathing is a passive activity for the most part.
We usually don't think about it. You do it in your sleep. Breathing happens because the carbon dioxide level in your blood becomes high, triggering a signal from your brain to your diaphragm to contract. You don't consciously contract your diaphragm just like you don't consciously contract your heart muscle. Your diaphragm is a flexible shelf-like sheet of muscle totally inside your body.