The reason for this study is to see if the study drug selpercatinib compared to a standard treatment is effective and safe in participants with rearranged during transfection (RET) fusion-positive non-squamous non-small cell lung cancer (NSCLC) that has spread to other parts of the body. Participants who are assigned to the standard treatment and discontinue due to progressive disease have the option to potentially crossover to selpercatinib.