Inclusion Criteria:  STEP 0 INCLUSION CRITERIA Histologically proven mucosal melanoma by local pathology Central PD-L1 tumor tissue submission STEP 1 INCLUSION CRITERIA Receipt of the central PD-L1 testing results available  Disease status-Resected R0 or R1 disease patients. Patients eligible for randomization have resected R0 or R1 disease (with negative margins or positive microscopic margins) that must meet one of the following 4 criteria as defined below:  Regional lymph node (LN) involvement; OR In-transit metastases/satellite primary disease; OR  Single localized, primary disease meeting one of the following site-specific requirements:  Head/neck - any primary lesion if sinonasal; pT4a or above for nasal or oral cavity, given slightly improved OS Anorectal - any primary lesion Vaginal/cervical - any primary, as they have 5 year OS rates of 5-25 Urinary tract - any primary urethral or bladder tumor Penile Vulvar- AJCC cutaneous stage IIB or higher Esophageal/gallbladder - any primary Locoregionally recurrent following prior resection, meeting at least one of the above criteria In addition, patients must have undergone cross-sectional imaging of the brain, chest, abdomen and pelvis with no evidence of distant metastatic disease  Disease status-Non-resected R2 or metastatic disease patients  Non-resected R2 or metastatic disease that is assessable and measurable radiographically or by physical examination  Prior Treatment:  No prior systemic checkpoint inhibitor therapy of mucosal melanoma, including in the adjuvant setting, is allowed. Prior adjuvant chemotherapy or interferon is allowed. No other active, concurrent malignancy that requires ongoing systemic treatment or interferes with radiographic assessment of melanoma response as determined by the investigator. Any radiation must have completed 28 days prior to randomization and the patient must have adequately recovered from its effects. Surgery must have completed 28 days prior to randomization. Not pregnant and not nursing, because this study has an agent that has known genotoxic, mutagenic and teratogenic effects. Therefore, for women of childbearing potential only, a negative pregnancy test done =< 7 days prior to registration is required Age >= 18 years Eastern Cooperative Oncology Group (ECOG) performance status 0-2 Absolute neutrophil count (ANC) >= 1,500/mm^3 Platelet count >= 100,000/mm^3 Creatinine =< 1.5 x upper limit of normal (ULN) OR creatinine clearance (CrCl) >= 50mL/min/1.73 m^2 for patients with creatinine levels above institutional normal Albumin >= 2.8 g/dL Lipase < 2.0 x ULN and no radiologic or clinical evidence of pancreatitis Urine protein/creatinine =< 1 Total bilirubin =< 1.5 x upper limit of normal (ULN) Aspartate aminotransferase (AST)/alanine aminotransferase (ALT) =< 2.5 x upper limit of normal (ULN)  No cardiovascular disease, including:  No history of acute coronary syndromes (including myocardial infarction and unstable angina), coronary artery bypass graft (CABG) coronary angioplasty, or stenting within 6 months prior to study entry. No history of current class II or higher congestive heart failure as defined by the New York Heart Association (NYHA) functional classification system. No refractory hypertension defined as a blood pressure of systolic > 140 mmHg and/or diastolic > 90 mmHg despite adequate attempts at anti-hypertensive therapy. No history of myocarditis. No history of syncope of cardiovascular etiology, uncontrolled cardiac arrhythmia, history of Mobitz II second degree or third degree heart block without a permanent pacemaker in Association (NYHA) class II to IV heart failure, or stroke/transient ischemic attack (TIA) within the past 3 months. No corrected QT interval by Fridericia's formula (QTcF) > 500 msec. Note: if initial QTcF is found to be > 500 ms, two additional EKGs separated by at least 3 minutes should be performed. If the average of these three consecutive results for QTcF is =< 500 ms, the subject meets eligibility in this regard.  No underlying hematologic issues, including:  Congenital bleeding diathesis Gastrointestinal (GI) bleeding requiring intervention within the past 6 months, unless directly related to mucosal melanoma Active hemoptysis within 42 days prior to study enrollment. Active tumor lesions with cavitations or tumor lesions which invade, encase, or abut major blood vessels. The anatomic location and characteristics of primary tumors or metastases as well as the medical history should be carefully reviewed in the selection of subjects for treatment with cabozantinib/placebo. Pulmonary emboli or deep vein thromboses (DVT) that require an active anticoagulation regimen. No known or suspected history of cytopenia (low white blood cell [WBC], hemoglobin or platelet count) of greater than 3 months duration with an unknown cause, myelodysplastic syndrome, or hematologic malignancies. No clinical, laboratory or radiographic evidence of an active bacterial, fungal, or viral infection requiring treatment at the time of pre-registration (e.g., active symptoms of COVID-19 infection or a post-infectious symptomatic autoimmune syndrome, serious bacterial infections requiring antibiotics). No known or suspected gastrointestinal disorder affecting absorption of oral medications.  Comorbid conditions:  No active autoimmune disease or any condition requiring systemic treatment with either corticosteroids (> 10 mg daily of prednisone equivalents) or other immunosuppressive medications within 14 days of study drug administration. Inhaled or topical steroids and adrenal replacement doses > 10 mg daily prednisone equivalents are permitted in the absence of active autoimmune disease. No history of autoimmune motor neuropathy (e.g., Guillain-Barre syndrome, myasthenia gravis) or non-infectious pneumonitis. No history of severe allergic reactions to an unknown allergen or any components of the study drugs or its excipients. No history of gastrointestinal perforation or abdominal fistula.  No clinically suspected central nervous system (CNS) (leptomeningeal or parenchymal) metastases. Patients with a history of CNS metastasis(s) will be allowed as long as  The metastatic site(s) were adequately treated as demonstrated by clinical and radiographic improvement, AND The patient has recovered from the intervention (no residual adverse events > Common Terminology Criteria for Adverse Events [CTCAE] grade 1), AND The patient has remained without occurrence of new or worsening CNS symptoms for a period of 28 days prior to enrollment. No history of seizure or any condition that may increase the patient's seizure risk (e.g., prior cortical stroke, significant brain trauma) within 2 years. No clinically active or chronic liver disease resulting in moderate/severe hepatic impairment (Child-Pugh class B or C), ascites, coagulopathy or bleeding due to liver dysfunction. No untreated spinal cord compression or evidence of spinal metastases with a risk of impending fracture or spinal cord compression. Spinal metastases must have completed planned radiation or surgical therapy prior to registration.  Concomitant medications:  Chronic concomitant treatment with strong inhibitors of CYP3A4 is not allowed on this study. Patients on strong CYP3A4 inhibitors must discontinue the drug for 14 days prior to registration on the study. Chronic concomitant treatment with strong CYP3A4 inducers is not allowed. Patients must discontinue the drug 14 days prior to the start of study treatment.