Singer-songwriter Billie Eilish is just beginning to know herself. With her latest album, “Hit Me Hard and Soft,” and her current tour (her first without brother Finneas and her parents), the music superstar (soon to turn 23) is discovering parts of herself she didn’t know were there. She describes to correspondent Anthony Mason finding a new voice as a songwriter, and about stretching her singing after starting vocal lessons. [Watch more from Mason’s interview with Eilish on “CBS Mornings” December 10.]