Fianlimab/Cemiplimab

This study is researching an experimental drug called fianlimab (also called REGN3767), combined with a medication called cemiplimab compared against cemiplimab combined with placebo (a placebo looks like a treatment but does not contain any real medicine), collectively called "study drugs" in this form.
The study is focused on participants with head and neck cancers who have not been previously treated for head and neck cancer that has come back or spread to other parts of the body, referred to as recurrent or metastatic (R/M) head and neck squamous cell carcinoma (HNSCC).
The study is looking at several other research questions, including:
* What side effects may happen from taking the study drugs
* How much of each study drug is in the blood at different times
* Whether the body makes antibodies against the study drug(s) individually (which could make the study drugs less effective or could lead to side effects)
* Compatible research to better understand the study drugs and HNSCC

The researchers are doing this study to find out whether stereotactic body radiation therapy (SBRT) in combination with immunotherapy (cemiplimab with or without fianlimab) before cystectomy is an effective and safe treatment for people with muscle-invasive bladder cancer (MIBC).