This study looks at how adding two drugs, mosunetuzumab and polatuzumab vedotin, can help patients with certain types of blood cancer called diffuse large B-cell lymphoma. This is for patients whose disease has come back or hasn't responded to other treatments, including a special therapy called CAR T-cell therapy. CAR T-cell therapy involves changing a patient’s immune cells (T-cells) in a lab to better fight cancer. Mosunetuzumab and polatuzumab vedotin are medicines that can target and kill cancer cells. Patients in this study will be randomly assigned to different groups: one group receives mosunetuzumab, another gets polatuzumab vedotin, a third group gets both drugs, and the last group is observed without these drugs. The study aims to see if these drugs can improve survival and reduce cancer. It will also look at possible side effects.

• The study includes multiple visits and tests.
• Patients may receive additional cancer drugs after CAR T-cell therapy.
• Participants will be monitored for treatment effects and side effects.