The purpose of this study is to evaluate an experimental drug called RMC-6236 in pancreatic cancer patients whose cancer has spread outside the pancreas (metastatic) and whose cancer has returned after treatment or did not respond to treatment. Study procedures include physical exams, questionnaires, collection of blood and urine samples, echocardiogram, electrocardiograms, computed tomography scans, biopsies, and administration of study drugs. Participants will be randomly assigned to receive either RMC-6236 or a chemotherapy regimen chosen by the study doctor (combinations of approved drugs 5-fluorouracil, leucovorin, oxaliplatin, irinotecan, and liposomal irinotecan or gemcitabine and nab-paclitaxel); participants have a 50% chance of being assigned to either group. Researchers will examine the effect of RMC-6236 or chemotherapy on the patient’s cancer.