Additional Study Information: Can we lower the chance of Diffuse Large B-cell Lymphoma (DLBCL) cancer growing or spreading by adding treatment to the usual therapy? This study is being done to find out if this approach is better or worse than the usual approach for DLBCL patients receiving chimeric antigen receptor (CAR) T-cell therapy. The usual approach for Relapsed/Refractory DLBCL patients who have a partial response or stable disease after CAR T-cell therapy is follow-up visits with no further therapy. Some patients will improve their response over time, but many eventually will experience relapse. This study will test additional treatments after CAR T-cell to try and improve responses. This study will compare the usual treatment alone to using the study drugs mosunetuzumab and/or polatuzumab vedotin after CAR T-cell therapy. The addition of these study drugs to the usual treatment could help shrink or control the cancer. But, it could also cause side effects. This study will help the study doctors find out if this different approach is better, the same, or worse than the usual approach. To decide if it is better, the study doctors will look to see if the study approach lowers the chance of cancer coming back or getting worse after CAR T-cell therapy compared to the usual approach. Mosunetuzumab is an experimental drug known as a bispecific antibody targeting CD20 and CD3. This antibody brings the T-cells of the immune system and the lymphoma cells together so that the immune system can potentially destroy the lymphoma cells. Polatuzumab vedotin is an antibody-drug conjugate which targets CD79b, a molecule that is on the surface of B-cell lymphoma cells. Antibody-drug conjugates consist of an antibody against the tumor cell that is attached to a chemotherapy drug. The antibody brings the chemotherapy drug to the lymphoma cell so that it can potentially destroy the lymphoma cell.