The purpose of this study is to assess the anti-tumor activity of amivantamab as a monotherapy (Cohorts A, B, and C), to assess the recommended phase 2 combination dose (RP2CD) of amivantamab when added to SoC chemotherapy (Ph1b cohorts) and to characterize the safety of amivantamab when added to standard-of care (SoC) chemotherapy in participants with metastatic colorectal cancer (mCRC) (Ph2 cohorts).