I don't think it's a binary switch between "understanding" and "not understanding". I have the basic gist of how semiconductors and logic gates work, I know a little about how CPUs and assembly work, and I can work with assembly if I have to, but those aren't my areas of expertise. I know enough about floating point arithmetic that I can identify floating point errors as floating point errors, but I don't claim to have anything close to the fluency in those systems that I do for higher-level languages. The ability to specialize makes it possible to make fantastic machines like the global Internet even though no one person on earth understands all the sub-components to the degree that a specialist in a particular sub-component does. I'm not saying that there aren't some computing systems that are fully comprehended by a single person, but the ability to specialize increases the scope of what is collectively possible.