The purpose of this study is to test an experimental drug called RMC-6236. RMC-6236 is an experimental drug, which means that it is not approved by the United States (US) Food and Drug Administration (FDA). RMC-6236 is currently being investigated in other ongoing studies, and this is the first study investigating RMC-6236 in only pancreatic cancer. You will be randomly assigned to receive either RMC-6236 or a chemotherapy regimen chosen by the study doctor (in the form of combinations of approved drugs 5-fluorouracil, leucovorin, oxaliplatin, irinotecan, and liposomal irinotecan or...