This study is researching an experimental drug called REGN3767, also known as fianlimab (R3767), when combined with another medication called REGN2810, also known as cemiplimab (each individually called a "study drug" or called "study drugs" when combined). The study is focused on patients with a type of skin cancer known as melanoma. The aims of the study are to see how effective the combination of fianlimab and cemiplimab are in treating the melanoma skin cancer, in comparison with a medication, pembrolizumab, approved for the treatment of melanoma skin cancer in adults, and to observe any similarities, or differences, in how the study drugs work in adolescent participants compared with adult participants. The study is looking at several other research questions, including: * What side effects may happen from receiving the study drugs * How much study drug is in the blood at different times * Whether the body makes antibodies against the study drugs (which could make the drugs less effective or could lead to side effects). Antibodies are proteins that are naturally found in the blood stream that fight infections. * How administering the study drugs might improve quality of life