The purpose of this study is to test the good and bad effects of a drug called M3814 in combination with the drug avelumab and radiation therapy for advanced/metastatic solid tumors and hepatobiliary malignancies.

M3814 in combination with avelumab and radiation therapy is considered investigational because it is not yet approved by the U.S. Food and Drug Administration (FDA) for this use. Although M3814 and avelumab have been used to treat cancer, it is not yet known if the combination of M3814 with avelumab and radiation therapy will work to treat advanced/metastatic hepatobiliary cancer.

Participants who take part in this study will be randomized and either get radiation therapy followed by a combination of avelumab and M3814, or radiation therapy followed by avelumab. Avelumab is given via a vein in the arm and M3814 is taken orally via tablets.

Participants will receive study treatment until their disease gets worse, or the side effects become too severe. After finishing the combination of M3814 and avelumab, the study doctor and team will continue to monitor participants for up to 5 years and watch for side effects