Inclusion Criteria:

* Have a susceptible fibroblast growth factor receptor (FGFR) mutation or fusion either by urine testing or tumor tissue testing (from TURBT tissue), as determined by central or local testing

* Participants must be willing to undergo all study procedures (e.g., multiple cystoscopies from Screening through the end of study and TURBT for assessment of recurrence/progression) and receive the assigned treatment, including intravesical chemotherapy if randomized into that arm.

* Visible papillary disease must be fully resected prior to randomization and absence of disease must be documented at Screening cystoscopy. The same method for visualizing disease at Screening cystoscopy should be used throughout for the participant (white light versus enhanced assessment method)

* Can have a prior or concurrent second malignancy (other than the disease under study) which natural history or treatment is unlikely to interfere with any study endpoints of safety or the efficacy of the study treatment

* Have an Eastern Cooperative Oncology Group performance status of 0 to 2

Exclusion Criteria:

* Known allergies, hypersensitivity, or intolerance to any study component or its excipients, including: a. Erdafitinib excipients; b.TAR-210 drug delivery system constituent materials ; c. urinary placement catheter materials; d. MMC or chemically related drugs; e. Gemcitabine or chemically related drugs

* Presence of any bladder or urethral anatomic feature (that is, urethral stricture) that, in the opinion of the investigator, may prevent the safe insertion, indwelling use, removal of TAR-210 or passage of a urethral catheter for intravesical chemotherapy

* Polyuria with recorded 24-hour urine volumes greater than (>) 4000 mL

* Current indwelling urinary catheters, however, intermittent catheterization is acceptable

* Had major surgery or had significant traumatic injury and/or not fully recovered within 4 weeks before first dose (TURBT is not considered major surgery)