At 32, Selena Gomez is an accomplished actor (including in the new movie “Emilia Pérez”), successful singer-songwriter, billionaire business owner, and philanthropist. But her gutsiest move may have been sharing herself. She talks with correspondent Tracy Smith about her mother’s inspiration; her relationship with Steve Martin and Martin Short, her co-stars in “Only Murders in the Building”; and why she went public with her health struggles and bipolar diagnosis in the documentary “My Mind and Me,” saying, “One of the strongest things you can do is be vulnerable.”