This randomized, multi-site, three-part study will test a new treatment called BNT314, which is designed to help the body's immune system fight cancer in combination with another new treatment (BNT327, which is an immune checkpoint inhibitor) and chemotherapy in participants with metastatic colorectal cancer (mCRC). This study will enroll participants with microsatellite stable or mismatch repair proficient (MSS/pMMR) mCRC who did not respond well to their first schema of chemotherapy. In one part of the study (i.e., Part B) mCRC participants will be enrolled, who have not received any systemic therapy before for their cancer.