The purpose of this study is to: (a) characterize the safety and tolerability of and to identify the recommended Phase 2 dose (RP2D) and schedule for erdafitinib in combination with cetrelimab, and for erdafitinib in combination with cetrelimab and platinum (cisplatin and carboplatin) chemotherapy and; (b) to evaluate the safety and clinical activity of erdafitinib alone and in combination with cetrelimab in cisplatin-ineligible participants with metastatic or locally advanced urothelial cancer (UC) with select fibroblast growth factor receptor (FGFR) gene alterations and no prior systemic therapy for metastatic disease.