MagnetisMM-32: A Study to Learn About the Study Medicine Called Elranatamab in People With Multiple Myeloma (MM) That Has Come Back After Taking Other Treatments (Including Prior Treatment With an Anti-CD38 Antibody and Lenalidomide)

The purpose of this study is to learn about the study medicine called elranatamab.This study aims to compare elranatamab to other medicines for the treatment of MM (a type of cancer). This study is seeking participants who: * Are 18 years of age or older and have MM. * Have received treatments before for MM. * Have MM that has returned or not responded to their most recent treatment. Half of the participants will receive elranatamab. The other half of participants will receive a combination therapy selected by the study doctor. The selected combination therapy will include 2 to 3 different medicines commonly used to treat MM. Elranatamab will be given as a shot under the skin at the study clinic about once a week. This may change to a smaller number of shots later in the study. The medicines in the combination therapy will be taken by mouth (at home or at the study clinic) AND will be given either as: * a shot under the skin at the study clinic * through a needle in the vein at the study clinic The number of times these medicines will be taken depends on what combination therapy the study doctor selects. Participants may continue to receive elranatamab or a combination therapy until their MM is no longer responding. The study team will see how each participant is doing with the study treatment during regular visits at the study clinic. The study team will continue to follow-up with participants after study treatment with telephone contacts (or visits). The study will compare the experiences of people receiving elranatamab to those people receiving a combination therapy. This will help learn about the safety and how effective elranatamab is.