U3-1402 in Metastatic or Unresectable Non-Small Cell Lung Cancer

The primary objectives are: * For Dose Escalation, to assess the safety and tolerability of HER3-DXd in the study population and to determine the recommended dose for expansion (RDE) of HER3-DXd in the study population * For Dose Expansion, to investigate the antitumor activity of HER3-DXd * For Cohort 5, investigate the antitumor activity of HER3-DXd in participants with locally advanced or metastatic NSCLC whose tumors harbor a KRAS-G12C mutation after the failure of targeted therapy The number of treatment cycles is not fixed in this study. Participants will continue study treatment (for approximately 36 months) until they decide not to (withdraw consent), their disease gets worse \[progressive disease (PD)\], or side effects become unacceptable (unacceptable toxicity) or other stopping reasons have been met.