Billie Eilish and Finneas chat with ET’s Kevin Frazier backstage at the 66th Annual GRAMMY Awards, which aired Sunday on CBS and Paramount+. Billie reacts to winning Song Of The Year for 'What Was I Made For?' and explains why she thanked Greta Gerwig in her acceptance speech. Plus, she reveals what Miley Cyrus said to her after winning Record of the Year for 'Flowers.'