• Histologically confirmed MIUC (also termed transitional cell carcinoma \[TCC\]) of the bladder

• Tumor, nodes, and metastases (TNM) classification (based on American Joint Committee on Cancer \[AJCC\] Cancer Staging Manual, 8th Edition; Amin et al. 2016) at pathological examination of surgical resection specimen as follows: For participants treated with prior neoadjuvant chemotherapy (NAC): tumor stage of ypT2-4a or ypN+ and M0. For participants who have not received prior NAC: tumor stage of pT2-4a or pN+ and M0

• Surgical resection of MIUC of the bladder

• Participants who have not received prior platinum-based NAC must be ineligible for cisplatin-based adjuvant chemotherapy, have refused it, or will not receive it based on physician's decision

• ctDNA assay developed based on tumor tissue specimen and matched normal DNA from blood

• Tumor programmed death ligand (PD-L1) expression per immunohistochemistry (IHC) that is evaluable by central testing of a representative tumor tissue specimen

• Absence of residual disease and absence of metastasis, as confirmed by a negative baseline computed tomography (CT) or magnetic resonance imaging (MRI) scan of the pelvis, abdomen, and chest no more than 4 weeks prior to enrollment

• Full recovery from cystectomy and enrollment within 24 weeks following cystectomy. Minimum of 6 weeks must have elapsed from surgery

⁃ Additional Inclusion Criteria for the Treatment Phase:

• Blood for plasma ctDNA sample evaluated to be ctDNA positive, defined as the presence of two or more mutations out of the 16 mutations identified based on participant' whole exome sequencing (WES) evaluable (ctDNA assay designability) report

• Absence of residual disease and absence of metastasis, as confirmed by a negative baseline CT or MRI scan of the pelvis, abdomen, and chest no more than 28 days prior to randomization, as assessed by the investigator and Independent Review Facility

• Eastern cooperative oncology group (ECOG) performance status of ≤ 2

• Life expectancy ≥12 weeks

• Adequate hematologic and end-organ function

• For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraception and agreement to refrain from donating eggs