1. 18 years or older and able to sign informed consent and comply with the protocol. 2. Measurable disease as defined by RECIST V1.1 criteria for solid tumors. 3. 3. Part A and Part B: Histologically or cytologically confirmed unresectable advanced or metastatic solid gastric, gastroesophageal junction (GEJ), biliary tract or pancreatic carcinomas previously treated for advanced (metastatic or unresectable) disease or for which treatment is not available or not tolerated. Part C, substudy C1: 2L m/a GC/GEJ-C patients will receive Spevatamig (PT886) in combination with Paclitaxel. Patients who are HER2 positive are eligible. Part C, substudy C2: 1L m/a PDAC patients will receive Spevatamig (PT886) in combination with Gemcitabine plus nab-Paclitaxel (Abraxane). Part C, substudy C3: 1L m/a PDAC patients will receive Spevatamig (PT886) in combination with Gemcitabine plus FOLFIRINOX/mFFX. Part C, substudy C4: Patients with m/a BTC who have progressed on 1L SOC chemotherapy (GemCis) ± ICI and are eligible for 2L SOC FOLFOX treatment. Part D, substudy D3: 2L or 3L m/a GC/GEJ-C patients will receive Spevatamig (PT886) in combination with KEYTRUDA® (pembrolizumab). Part D, substudy D4: 1L HER2 negative m/a GC/GEJ-C patients will receive Spevatamig (PT886) in combination with SOC chemotherapy and KEYTRUDA® (pembrolizumab). 4. Able to provide a formalin fixed, paraffin embedded (FFPE) tumor tissue sample (preferably fresh biopsy or if not possible, archival tissue) to be assessed for CLDN18.2 expression and other biomarkers. 5. ECOG performance status of 0 or 1. 6. Adequate organ function confirmed at screening and within 72 hours of initiating treatment.

Patients are excluded from the study if any of the following criteria apply: 1. Women who are pregnant or lactating. 2. Women of child-bearing potential (WOCBP) who do not use adequate birth control. 3. Has an active autoimmune disease that has required systemic treatment in the past 2 years. 4. Prior CLDN18.2 or CD47 targeting therapies, or SIRPα (signal regulatory protein alpha) targeting agents. Additional inclusion and exclusion criteria will apply.