Substudy 06C: A Study of Investigational Agents With Pembrolizumab (MK-3475) and Chemotherapy in Participants With First-Line Locally Advanced Unresectable/Metastatic Gastroesophageal Adenocarcinoma (MK-3475-06C/KEYMAKER-U06)

Inclusion Criteria: The main inclusion criteria include but are not limited to the following: * Has histologically and/or cytologically confirmed diagnosis of previously untreated locally advanced unresectable or metastatic first-line (1L) gastroesophageal adenocarcinoma * Is not expected to require tumor resection during the treatment course * Tumor tissue must be confirmed as negative for human epidermal growth factor receptor 2 (HER2) expression as classified by American Society of Clinical Oncology/College of American Pathologists (ASCO-CAP) guidelines * Core/excisional biopsy of a tumor lesion not previously irradiated has been provided * Participants who have adverse events (AEs) due to previous anticancer therapies must have recovered to ≤Grade 1 or baseline * Participants with endocrine-related AEs who are adequately treated with hormone replacement therapy are eligible * Has adequate organ function * Has measurable disease per Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST 1.1) as determined by the local site investigator/radiology assessment and verified by blinded independent central review (BICR) * Has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 within 3 days prior to the first dose of study intervention * Has a life expectancy of at least 6 months * Participants who are hepatitis B surface antigen (HBsAg) positive are eligible if they have received Hepatitis B Virus (HBV) antiviral therapy for at least 4 weeks, and have undetectable HBV viral load prior to allocation/randomization * Participants with history of Hepatitis C Virus (HCV) infection are eligible if HCV viral load is undetectable at screening * Human Immunodeficiency Virus (HIV)-infected participants must have well controlled HIV on antiretroviral therapy Exclusion Criteria: The main exclusion criteria include but are not limited to the following: * Has squamous cell or undifferentiated gastroesophageal cancer. * Has had previous therapy for locally advanced unresectable or metastatic gastric/gastroesophageal junction (GEJ)/esophageal adenocarcinoma * Has experienced weight loss \>20% over 3 months before the first dose of study intervention * Has a history of documented severe dry eye syndrome, severe Meibomian gland disease and/or blepharitis, or severe corneal disease that prevents/delays corneal healing * Has Grade ≥2 peripheral neuropathy * Has active inflammatory bowel disease requiring immunosuppressive medication or previous history of inflammatory bowel disease * Has uncontrolled, significant cardiovascular disease or cerebrovascular disease within 6 months preceding study intervention * Has accumulation of pleural, ascitic, or pericardial fluid requiring drainage or diuretic drugs within 2 weeks prior to enrollment * Has history of human immunodeficiency virus (HIV) infection with Kaposi's sarcoma and/or Multicentric Castleman's Disease * Has received prior treatment with a trophoblast antigen 2 (TROP2)-targeted or anti-human epidermal growth factor receptor 3 (HER3) targeted agents * Has received prior treatment with a topoisomerase I inhibitor-based antibody-drug conjugate (ADC) and/or a topoisomerase I inhibitor-based chemotherapy * Has received prior systemic anticancer therapy within 4 weeks before the first dose of study intervention * Has received prior therapy with an anti-Programmed Cell Death Protein 1 (PD-1), anti-Programmed Cell Death-Ligand 1 (PD-L1), anti-Programmed Cell Death-Ligand 2 (PD-L2) agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (TCR) * Has received prior radiotherapy within 2 weeks of start of study intervention, or has radiation related toxicities, requiring corticosteroids * Has received a live or live-attenuated vaccine within 30 days before the first dose of study intervention * Has received a strong inducer/inhibitor of CYP3A4 that cannot be discontinued * Has received an investigational agent or has used an investigational device within 4 weeks prior to study intervention administration * Has diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of study intervention * Has known additional malignancy that is progressing or has required active treatment within the past 3 years * Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis * Has Severe hypersensitivity (≥Grade 3) to pembrolizumab, sacituzumab tirumotecan, patritumab deruxtecan, or other biologic therapy, chemotherapy (ie, oxaliplatin, fluorouracil, capecitabine), leucovorin, levoleucovorin, or any of their excipients * Has active autoimmune disease that has required systemic treatment in the past 2 years * Has history of interstitial lung disease (ILD)/pneumonitis irrespective of steroid use, or current ILD, or suspected ILD, or ILD that cannot be ruled out by imaging at Screening * Has an active infection requiring systemic therapy * Has concurrent active hepatitis B (defined as Hepatitis B surface antigen \[HBsAg\] positive and/or detectable HBV DNA) and hepatitis C virus (defined as anti-hepatitis C virus \[HCV\] Ab positive and detectable HCV ribonucleic acid \[RNA\] infection or a known history of hepatitis B and/or C infection * Has history or current evidence of any condition, therapy, laboratory abnormality, or other circumstance that might confound the results of the study or interfere with the participant's ability to cooperate with the requirements of the study * Has gastrointestinal (GI) obstruction, poor oral intake, or difficulty in taking oral medication * Has poorly controlled diarrhea * Has had a major surgery or significant traumatic injury within 4 weeks before the first dose of study intervention * Has history of allogeneic tissue/solid organ transplant * Has not adequately recovered from major surgery or has ongoing surgical complications