The former question there is baaaarely touched upon in the show, you're right. It's pretty clear though that the Angels are supposed to be a force or nature in the sense that they don't have human minds(Fruit of Life vs. Fruit of Knowledge). Humans aren't able to comprehend the scope of the Angels because they exist on an entirely different conceptual level than any life found on Earth. For what the show is trying to do I feel like this unexplained force is pretty interesting/cool so it's fine the way it is, even as important as the questions they pose are. Again, the show is more trying to deal with the human reaction to them than explain what exactly they are(though yes they have deeper explanations in expanded materials).Like, why do angels attack humans? Why are they attacking NERV?
The latter is explained somewhat by the end though, the Angels are simply trying to get at what NERV has hidden away inside Terminal Dogma to initiate Instrumentality/recover Adam. It's why every conflict in the show is centered around Tokyo and they aren't just attacking any random city.
Again the show seems to gleam just enough info about the deeper conflict to keep it intrinsically relevant without making the show completely about the physical conflict itself.