Study of the Adverse Events and Change in Disease State of Pediatric Participants (and Young Adults Between the Ages of 18-25) With Relapsed/Refractory Aggressive Mature B-cell Neoplasms Receiving Subcutaneous (SC) Injections of Epcoritamab

Inclusion Criteria: * Participants \>= 1 and \< 18 years old at time of primary diagnosis with Burkitt's or Burkitt-like lymphoma/leukemia, diffuse large B-cell lymphoma (DLBCL), or other aggressive mature (CD20+) B-cell lymphomas. Participants up to 25 years of age with Burkitt's or Burkitt-like lymphoma/leukemia are also eligible. * Disease pathologically confirmed (tumor tissue) by local testing. * Relapsed or primary refractory disease meeting any of the following criteria: * Progressive disease at any time during second-line chemoimmunotherapy (CIT). * Best response of stable disease (SD) after a minimum of 2 cycles of second-line CIT. * Best response of partial response (PR) after a minimum of 3 cycles of second-line CIT. * Complete Response (CR) after a minimum of 3 cycles of second-line CIT therapy but unfit or ineligible for consolidation with cell therapy. * Not in CR and unable to initiate or tolerate (i.e., must discontinue) second-line CIT. * Have received cell therapy (allogeneic or autologous transplant or chimeric antigen receptor T-cell (CAR-T) therapy) as consolidation but have not obtained or maintained a CR. * Recovery from toxic effects of prior chemoimmunotherapy. * Performance status by Lansky (\< 16 years old at evaluation) or Karnofsky (\>= 16 years old at evaluation) score \>= 50 or Eastern Cooperative Oncology Group (ECOG) score \<= 2 . * Adequate bone marrow, hepatic, and renal function. Exclusion Criteria: * Known central nervous system (CNS) involvement by lymphoma at screening as confirmed by screening magnetic resonance imaging (MRI)/computed tomography (CT)/positron emission tomography (PET) brain scans (participants with evidence of CNS disease only in the cerebrospinal fluid (CSF) will be eligible). * Other malignancy requiring therapy. * Currently receiving anti-cancer therapy, including chemotherapy (excluding intrathecal therapy), radiotherapy, small molecules, monoclonal antibodies, cell therapy, or other investigational agents.