The purpose of the study is to compare how the new combination treatment (Sigvotatug Vedotin plus pembrolizumab) works compared to pembrolizumab alone in patients with non-small cell lung cancer (NSCLC) with high levels of PD-L1. This is a protein that acts as a kind of "brake" to keep the body's immune responses under control. The study is seeking for participants who: * Are confirmed to have NSCLC (Stage 3 or 4). * Have PD-L1 levels in more than 50% of the cancer cells. All participants in this study will receive pembrolizumab at the study clinic once every 6 weeks as an intravenous (IV) infusion (give directly into a vein). In addition, half of the participants will also receive Sigvotatug Vedotin once every 2 weeks as an IV infusion in addition to receiving pembrolizumab. Participants may receive pembrolizumab for up to about two years. Those participants taking Sigvotatug Vedotin can continue until their NSCLC is no longer responding. The study team will monitorsee how each participant is doing with the study treatment during regular visits at the clinic.