Testing the Addition of Nivolumab to Standard Treatment for Patients With Metastatic or Unresectable Colorectal Cancer That Have a BRAF Mutation

Inclusion Criteria: * Participants must have a histologically or cytologically confirmed diagnosis of adenocarcinoma of the colon or rectum. The date of diagnosis will be determined according to the pathologic date of diagnosis * Participants must have measurable disease according to RECIST1.1 criteria. Computed tomography (CT) scans or magnetic resonance imaging (MRIs) used to assess measurable disease must have been completed within 28 days prior to registration. CT scans or MRIs used to assess non-measurable disease must have been completed within 42 days prior to registration. All disease must be assessed and documented on the Baseline Tumor Assessment Form * Participants must have documented unresectable and/or metastatic disease on CT or MRI imaging. All disease must be assessed and documented on the Baseline Tumor Assessment Form * Participants must have BRAF\^V600E mutated colorectal cancer as tested in a Clinical Laboratory Improvement Act (CLIA)-certified laboratory * Participants must have proficient mismatch repair (pMMR) or microsatellite stable (MSS) status as tested in a CLIA-certified laboratory and documented by the treating clinician. Proficient mismatch repair status can be determined by intact expression by immunohistochemistry of all 4 mismatch repair proteins (MLH1, MSH2, MSH6, and PMS2). Microsatellite instability can be determined by polymerase chain reaction (PCR) * Participants with brain metastases must have completed surgery or radiation therapy \…