Researchers are looking for new ways to treat people with high-risk muscle-invasive urothelial carcinoma (MIUC). Urothelial carcinoma is a type of bladder cancer that begins in cells that line the inside of the bladder and other parts of the urinary tract, such as part of the kidneys, ureters, and urethra. People with MIUC usually have chemotherapy before surgery, then surgery to remove the cancer. Chemotherapy is a type of medicine to destroy cancer cells or stop them from growing. After surgery, some people receive more treatment to prevent cancer from returning. Pembrolizumab is an immunotherapy, which is a treatment that helps the immune system fight cancer. Enfortumab vedotin (EV) is an antibody drug conjugate (ADC). An ADC attaches to a protein on cancer cells and delivers treatment to destroy those cells. Researchers want to learn if giving intismeran autogene (the study treatment) with pembrolizumab can prevent MIUC from returning after surgery. Intismeran autogene (also called mRNA-4157) is designed to treat each person's cancer by helping the person's immune system identify and kill cancer cells based on certain proteins found on those cancer cells. The goals of this study are to learn if people who receive intismeran autogene and pembrolizumab are alive and cancer free longer than those who receive placebo and pembrolizumab, and to learn about the safety of intismeran autogene, pembrolizumab, and EV, and if people tolerate them.