Additional Study Information: The purpose of this study is to determine the combination dose of patritumab deruxtecan and osimertinib that can be given to patients safely and to understand how well patritumab deruxtecan alone or in combination with osimertinib works to treat lung cancer. Osimertinib is an EGFR TKI therapy that targets EGFR mutations. Osimertinib has already been approved by several regulatory authorities for the treatment of patients with advanced non-small cell lung cancer whose tumors contain specific EGFR mutations. This will be the first time patritumab deruxtecan is used in combination with osimertinib in humans. Patritumab deruxtecan is an investigational drug being developed to treat patients with non-small cell lung cancer, as well as patients with breast cancer or colorectal cancer and it has not yet been approved by the FDA, so it is only used in research. Patritumab deruxtecan is an antibody drug conjugate (ADC), which is a monoclonal antibody that has chemotherapy attached to it. A monoclonal antibody is a molecule that is designed to target specific cancer cells, but it might also affect normal cells in the body. Chemotherapy is a type of cancer treatment that kills cells that grow and divide.