One person can, at least in principle, understand every last thing in a nuclear reactor. A few elements have roles at different levels of abstraction. And of course they have computers in them that invariably do stuff radically simpler than they could be doing.

But nobody will ever understand everything about a natural cell. Levels are an ad hoc phenomenon adhered to in varying degrees in certain places to contain variation. But few elements have only one role at one identifiable level, and you can never be sure one doesn't have another. And huge amounts of apparatus might radically change behavior in a subtly different environment.