I crossed mountains and seas. What song is it?

Ordinary road. The lyrics "I have crossed the mountains and seas" come from the song "Ordinary Road", written by Pu Shu and Han Han, composed and sung by Pu Shu. It's the theme song of the movie See you later. Included in the album Orion.

Lyrics excerpt: I crossed the mountains and seas, and I also crossed the sea of people. Everything I once owned suddenly drifted away like smoke.