The purpose of this study aims to determine if BT8009, when given in combination with pembrolizumab, works against urothelial cancer. This will be compared to standard chemotherapy treatment followed by avelumab therapy. This research study also aims to understand the side effects of BT8009 when given in combination with pembrolizumab. The study drug BT8009 is investigational (i.e., experimental). An investigational drug is one that has not been approved by the Food and Drug Administration (FDA) to treat a certain condition or illness or a certain type of patient. Pembrolizumab is an antibody...