This phase II trial compares the effect of adding radiation therapy to an immunotherapy drug called atezolizumab vs. atezolizumab alone in treating patients with urothelial cancer that has spread to other places in the body (metastatic). The addition of radiation to immunotherapy may shrink the cancer, but it could also cause side effects. Immunotherapy with monoclonal antibodies such as atezolizumab, may help the body's immune system attack the cancer, and may interfere with the ability of tumor cells to grow and spread. Stereotactic body radiation therapy (SBRT) is a type of radiation therapy that uses high energy x-rays to kill tumor cells and shrink tumors. This method uses special equipment to position a patient and precisely deliver radiation to tumors with high precision. This method may kill tumor cells with fewer doses over a shorter period and may cause less damage to normal tissue than conventional radiation therapy. The combination of atezolizumab and radiation therapy may be more efficient in killing tumor cells.