Inclusion Criteria:

          -  Histologic demonstration of transitional cell carcinoma of the urothelium. Variant
             urothelial carcinoma histologies such as glandular or squamous differentiation, or
             evolution to more aggressive phenotypes such as sarcomatoid or micropapillary change
             are acceptable

          -  Metastatic or locally advanced urothelial cancer

          -  Must have measurable disease by radiological imaging according to the Response
             Evaluation Criteria in Solid Tumors (RECIST, version 1.1) at baseline

          -  Prior systemic therapy for metastatic urothelial cancer: (a) For Phase 1b erdafitinib
             + cetrelimab cohort: Any number of lines of prior therapy; (b) For Phase 1b
             erdafitinib + cetrelimab + platinum chemotherapy cohort: No prior systemic therapy for
             metastatic disease; and renal function for participants must have a creatinine
             clearance (CrCl) greater than (>) 30 milliliter per minute (mL/min) to receive
             carboplatin and >60 mL/min to receive cisplatin as calculated by Cockcroft Gault and
             (c) Phase 2: No prior systemic therapy for metastatic disease and cisplatin-ineligible
             based on: ECOG PS 0-1 and at least one of the following criteria: Renal function
             defined as creatinine clearance (CrCl) less than (˂) 60 mL/min as calculated by
             Cockcroft-Gault; Grade 2 or higher peripheral neuropathy per NCI-CTCAE version 5.0;
             Grade 2 or higher hearing loss per NCI-CTCAE version 5.0 OR ECOG PS 2

          -  Eastern Cooperative Oncology Group (ECOG) performance status (PS) grade of: (a) Phase
             1b erdafitinib + cetrelimab cohort: ECOG 0-2; (b) Phase 1b erdafitinib + cetrelimab +
             platinum chemotherapy cohort: ECOG 0-1 for cisplatin and 0-2 for carboplatin (c) Phase
             2: ECOG 0-2

        Exclusion Criteria:

          -  Treatment with any other investigational agent or participation in another clinical
             study with therapeutic intent within 30 days prior to Cycle 1 Day 1. For Phase 1b,
             participants who have received the following prior antitumor therapy: received
             nitrosoureas and mitomycin C within 6 weeks

          -  Phase 1b erdafitinib + cetrelimab cohort: Chemotherapy within 3 weeks of Cycle 1 Day
             1; Phase 1b erdafitinib + cetrelimab + platinum chemotherapy cohort and Phase 2: Prior
             neoadjuvant/adjuvant chemotherapy is allowed if the last dose was given >12 months
             prior to recurrent disease progression and did not result in drug-related toxicity
             leading to treatment discontinuation

          -  Prior anti-programmed death receptor-1 (PD-1), anti-programmed death ligand-1 (PD-L1),
             or anti-programmed death ligand-2 (PD-L2) therapy. Prior neoadjuvant/adjuvant
             checkpoint inhibitor therapy is allowed if the last dose was given more than (>)12
             months prior to recurrent disease progression and did not result in drug-related
             toxicity leading to treatment discontinuation. PD-1 for non-muscle invasive bladder
             cancer is also allowed

          -  Active malignancies requiring concurrent therapy other than urothelial cancer

          -  Symptomatic central nervous system metastases