BGB-15025 Alone and in Combination With Anti-PD-1 Monoclonal Antibody Tislelizumab in Participants With Advanced Solid Tumors

Key Inclusion Criteria: 1. Phase 1a (dose escalation): Participants with histologically or cytologically confirmed advanced, metastatic, and unresectable solid tumors who have previously received standard systemic therapy or for whom treatment is not available, not tolerated or refused, and who have not received prior therapy targeting HPK1 2. Phase 1b (dose expansion): Participant with histologically or cytologically confirmed advanced, and metastatic including non-small cell lung cancer, and gastric/Gastroesophageal junction cancer that have no prior systemic treatment for advanced disease and esophageal squamous cancer who have progressed following systemic anticancer therapies 3. At least 1 measurable lesion as defined per RECIST 1.1. 4. Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 1 5. Adequate organ function as indicated by the following laboratory values up to first dose of study treatment: Hemoglobin≥ 90 g/L, Absolute neutrophil count ≥ 1.5 x 10\^9/L , Serum total bilirubin ≤ 1.5 x ULN (\< 3 x ULN for participants with Gilbert syndrome ), AST and ALT≤ 2.5 x ULN Key Exclusion Criteria: 1. Active leptomeningeal disease or uncontrolled and untreated brain metastasis. 2. Active autoimmune diseases or history of autoimmune diseases that may relapse 3. Any active malignancy ≤ 2 years before the first dose of study treatment except for the specific cancer under investigation in this study and any locally recurring cancer that has been treated with curative intent 4. Any condition that required systemic treatment with either corticosteroids (\> 10 mg daily of prednisone or equivalent) or other immunosuppressive medication ≤ 14 days before the first dose of study treatment 5. History of interstitial lung disease, noninfectious pneumonitis, or uncontrolled lung diseases including but not limited to pulmonary fibrosis, acute lung diseases, etc. 6. For Cohort A: Known actionable mutations (including but not limited to epidermal growth factor receptor (EGFR) gene, anaplastic lymphoma kinase (ALK) fusion oncogene, BRAF V600E, RET, MET, and ROS1, for which a targeted therapy has been approved by the local health authority and available. 7. For Cohort B: Diagnosed with G/GEJ adenocarcinoma with positive HER2 status NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.