This study is for people with a type of lung cancer called **metastatic non-small cell lung cancer (mNSCLC)**. The study compares two treatments: **rilvegostomig** and **pembrolizumab** for patients whose tumors have a protein called **PD-L1**. **PD-L1** is a protein that can help cancer hide from the immune system. **Eligibility**: Participants need to have stage IV mNSCLC, which means the cancer has spread. They must have specific tests showing no certain gene mutations, and be healthy enough to participate. **Exclusion**: People with other severe diseases, past organ transplants, or other cancers may not join. **Key Points**: - **Study Length**: Participants will be in this study for many weeks, with regular health checks. - **Visits Needed**: You will need to visit the study site multiple times for evaluations and treatments. - **Risks**: As with many cancer treatments, there may be side effects which will be monitored closely. This study aims to find out which treatment works better and is safer for patients with this specific type of lung cancer.