I've seen Arrival but there are things that are not explained. In the movie, the aliens come from somewhere using the spaceship called 'shell'. Where do they come from, from what planet/galaxy? And why are humans chosen to help them?
Nothing in the movie suggests where they came from or where they call home, so we don't know that part.
They came to us because they foresaw that they would need our help in the future. Because they can see that we were the ones who will help them, they came to us. This issue has been discussed before; probably one of the best explanations for the heptapods' motivations I've seen is Eldothiad's answer here: