Additional Study Information: The purpose of this study is to test a combination of 2 new drugs called RMC-6291 and RMC 6236 (the study drugs). RMC-6291 and RMC-6236 are new investigational (ie, experimental) drugs, which means that they are not approved by the United States (US) Food and Drug Administration (FDA) or any other health authorities. This is the first study in which RMC 6291 and RMC-6236 will be tested together in humans. This study will test the safety and anti-tumor effects of RMC-6291 and RMC-6236 when given together at different doses in participants with a change or alteration in the KRASG12C gene within the cells that make up their tumor. How a participant's body processes RMC-6291 and RMC-6236 when they are given together will also be tested. Cancer include: bladder cancer, breast cancer, colon and rectal cancer, esophageal cancer, gynecologic cancers, head and neck/oral cancer, kidney cancer, liver cancer, lung cancer, melanoma, pancreatic cancer, prostate cancer, stomach cancer, testicular cancer, and thyroid cancer.