Follicular lymphoma (FL) is the second most common B-cell cancer and the most common type of cancer of lymphocytes. Unfortunately, this disease is incurable with conventional treatment and the disease recurs in almost all patients. This study will assess how safe and effective epcoritamab is in combination with lenalidomide and rituximab (R2) in treating adult participants with previously untreated FL. Adverse events and change in disease condition will be assessed. Epcoritamab is an investigational drug being developed for the treatment of FL. Study doctors put the participants in 1 of 5 groups, called treatment arms. Each group receives a different treatment. Around 1095 adult participants with previously untreated FL will be enrolled in approximately 250 sites across the world. Participants will receive R2 (intravenous \[IV\] infusion of rituximab (R) and oral capsules of lenalidomide) alone or in combination with subcutaneous injections of epcoritamab. Participants may also receive investigator's choice chemoimmunotherapy (CIT): IV infusion of obinutuzumab (G) and IV injections of cyclophosphamide, IV injections of doxorubicin, IV injections of vincristine, oral tablets of prednisone (CHOP) \[G-CHOP\]/ R-CHOP or G and IV infusion of bendamustine (Benda) \[G-Benda\]/R-Benda. The total treatment duration will be 120 weeks for all arms except A2, which is 24 weeks of treatment. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.