"Autocomplete" is noy an abstraction level. It is the actual programmed behaviour.
At a certain level of abstraction, yes.
You can't understand human behaviour by reading a physics textbook.
Of course not. One of the major differences between intelligent and word-guessing autocomplete.
Do you think you could explain all AI behaviour by reading a physics textbook?
Nope. But someone smart enough could.
How the neural networks produce such surprisingly human characteristics is an open question with a ton of research going into it. Explaining this is a bit more than what one smart person can achieve.
With the same reasoning, human being are only a bunch of atoms, and the only reason they don't collide with other humans is because of the atomic force.
When your abstraction level is too low, it doesn't explain anything, because the system that is built on it is way too complex.