The setting sun belongs to mountains and seas, and the Milky Way belongs to dreams, which means that the setting sun will always set on the seashore or mountain side, and then the river where the stars gather will always enter their dreams. Then the superficial meaning of this sentence is like this. The superficial meaning of this sentence is also expressing a good artistic conception.

The sunset belongs to the mountains and seas, and the dream of the Milky Way means that its sunset glow is already beautiful, and then it gathers into a river like a star. If you can see it in a dream, it can also become a very beautiful scenery. And this is also the yearning for good things and the longing for a better life in the future.

Therefore, the superficial meaning and extended meaning of this sentence are these two kinds. This sentence also expresses the yearning for someone in the distance, or such a yearning for a couple in a different place.