Additional Study Information: The purpose of this study is to compare the effects, good or bad, of atezolizumab plus bevacizumab versus observation (not receiving any treatment) on patients with completely resected (surgically removed) or ablated (removed using heat) hepatocellular carcinoma (the most common type of liver cancer) who are at high risk for disease recurrence. Atezolizumab is an antibody (a protein similar to the ones produced by the body's immune system) that blocks the programmed death-ligand 1 (PD-L1) pathway. The PD L1 pathway is involved in regulating the body's natural immune response, but tumors can take advantage of this regulation to partially resist or evade the immune system. By blocking the PD-L1 pathway, atezolizumab may help the immune system stop or reverse the growth of tumors. Bevacizumab is an antibody that slows the growth of new blood vessels and may help the body stop the growth of tumors.