A Trial to Learn How Well Linvoseltamab Works Compared to the Combination of Elotuzumab, Pomalidomide and Dexamethasone for Adult Participants With Relapsed/Refractory Multiple Myeloma

This study is researching an experimental drug called linvoseltamab, also called REGN5458. Linvoseltamab has previously been studied by itself (without other cancer drugs) in participants who had advanced multiple myeloma that returned and needed to be treated again after many other therapies had failed. These participants were no longer benefiting from standard medications and had no good treatment options. In that study, some participants who were treated with linvoseltamab had improvement of their myeloma (shrinkage of their tumors), including some participants who had complete responses (that is, the treatment got rid of all evidence of myeloma in their bodies). This study is focused on participants who have multiple myeloma that has returned or needs to be treated again after one to four prior treatments and have standard cancer treatment options available to them. The aim of this study is to see how safe and effective linvoseltamab is compared to a combination of three cancer drugs: elotuzumab, pomalidomide and dexamethasone, (called EPd) in participants who have returned after having received prior treatment that included lenalidomide, a proteosome inhibitor, and (for participants in some countries) a cluster of differentiation 38 (CD38) antibody. Half of the participants in this study will get linvoseltamab, and the other half will get EPd. This study is looking at several other research questions, including: * How long participants benefit from receiving linvoseltamab compared with EPd * How many participants treated with linvoseltamab or EPd have improvement of their multiple myeloma and by how much * What side effects happen from taking linvoseltamab compared to EPd * How long participants live while receiving treatment or after treatment with linvoseltamab compared to EPd * If there is any improvement in pain after treatment with linvoseltamab compared to EPd