A Study to Assess Adverse Events of Intravenously (IV) Infused Etentamig (ABBV-383) in Adult Participants With Relapsed or Refractory Multiple Myeloma

Inclusion Criteria: * Must have measurable disease as outlined in the protocol. * Eastern Cooperative Oncology Group (ECOG) performance of \<= 2. Arm C and Arm D: ECOG performance of \<= 1. * Relapsed/refractory (R/R) multiple myeloma (MM) with documented evidence of progression during or after the participant's last treatment regimen based on the investigator's determination of the International Myeloma Working Group (IMWG) 2016 criteria. * Must be naïve to treatment with etentamig (ABBV-383). * Arm A: Must have received at least 3 or more lines of therapy, including a proteasome inhibitor (PI), an immunomodulatory imide drug (IMiD), and an anti-CD38 monoclonal antibody. * Arm B: Must have received at least 2 or more lines of therapy, including exposure to a PI, an IMiD, an anti-CD38 monoclonal antibody, and a prior B-cell maturation antigen (BCMA)-targeted therapy (must be an anti-drug conjugate \[ADC\] or chimeric antigen receptor T-cell \[CAR-T\] directed against BCMA). * Arm C: Must have received at least 2 or more lines of therapy, including exposure to a PI, an IMiD, and an anti-CD38 monoclonal antibody. Must be suitable for outpatient administration of etentamig (ABBV-383). * Arm D: Must have received at least 1 and no more than 3 prior lines of therapy, including exposure to a PI, an IMiD, or an anti-CD38 monoclonal antibody. Must be suitable for outpatient administration of etentamig (ABBV-383). Exclusion Criteria: * Arm A: Received BCMA-targeted therapy. * Arm C and Arm D: Rapidly progressing disease per investigator.