This study looks at patients with diffuse large B-cell lymphoma (a type of blood cancer) that has come back or not responded to treatment. It checks if two drugs, mosunetuzumab and polatuzumab vedotin, help after patients have had chemotherapy and a special treatment called CAR T-cell therapy. CAR T-cell therapy uses the patient's own immune cells to fight cancer. The study has four groups: one gets mosunetuzumab, another gets polatuzumab vedotin, the third gets both, and the last just gets watched. This trial aims to see if these drugs can keep cancer from growing further.

• The trial involves several visits for treatment and monitoring.
• Participants are randomly placed in one of four study groups.
• There may be risks, including side effects from the drugs.