This study is evaluating what the safest and most tolerable dose of enfortumab vedotin that can be taken in combination with the standard dose of erdafitinib for metastatic urothelial carcinoma. This study aims to find out if this approach is better or worse than the usual approach for metastatic bladder cancer. The usual approach is defined as care most people get for metastatic bladder cancer.

Enfortumab vedotin is approved by the U.S. Food and Drug Administration (FDA) and erdafitinib is approved by the FDA for patients with mutations in the FGFR 2/3 gene.

Participants will receive enfortumab vedotin and erdafitinib in combination for up to 2 years. Each cycle of treatment lasts 28 days. Erdafitinib will be taken orally daily and brought in with the participant on Days 15 and 17 of Cycle 1 and Day 1 of Cycle 2. The participant will receive enfortumab vedotin through a vein in the arm over 30 minutes on Days 1, 8, and 15 of each cycle. 

Participants will also undergo testing during study visits, including eye exams, blood collection, physical exams, imagining, pregnancy tests, and at the end of the study an Electrocardiogram (ECG).

Total study participation will last for up to 2 years. Follow-up will last for up to two years and occur with clinical visits every 3 months for 2 years.