Complex sentence: The movie ends with Liam and Billie are on stage (showing a montage of Josie and Liam finally getting married with Billie at their side) performing a song with Josie, Sam, Liam's dad, Jake, and the publicist watching.
Split into simpler sentences: The movie ends with Josie and Liam finally getting married with Billie at their side. It also ends with Liam and Billie are on stage, performing a song with Josie, Sam, Liam's dad, Jake, and the publicist watching.