Study to Evaluate the Safety, Pharmacokinetics, and Activity of GDC-1971 in Combination With Either Osimertinib in Participants With Unresectable, Locally Advanced, or Metastatic Non-Small Cell Lung Cancer, or With Cetuximab in Participants With Metastatic Colorectal Cancer

Inclusion Criteria: * Evaluable or measurable disease per RECIST v1.1 * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Life expectancy of ≥12 weeks * Adequate hematologic and organ function within 14 days prior to initiation of study Inclusion Criteria for Non-Small Cell Lung Cancer Cohorts * Histologically confirmed unresectable, locally advanced or metastatic adenocarcinoma of the lung that has progressed on/after prior treatment with third-generation epidermal growth factor receptor (EGFR) inhibitor (e.g., osimertinib) * Positive for an EGFR exon 19 deletion or exon 21 L858R mutation * Negative for acquired on-target EGFR alterations Inclusion Criteria for Colorectal Cancer Cohorts * Histologically confirmed metastatic adenocarcinoma of the colon or rectum that has progressed on/after prior treatment with an EGFR inhibitor (e.g., cetuximab or panitumumab) * Negative for kirsten rat sarcoma viral oncogene homolog (KRAS) alterations * Negative for neuroblastoma RAS viral oncogene homolog (NRAS) alterations * Negative for proto-oncogene B-Raf (BRAF) V600E alterations * In lieu of a fresh pre-treatment biopsy, a recently obtained biopsy performed after completion of osimertinib therapy will be acceptable Exclusion Criteria: * Treatment with chemotherapy, immunotherapy, biologic therapy, or an investigational agent as anti-cancer therapy within 3 weeks or 5 drug elimination half-lives, whichever is shorter, prior to initiation of study treatment * T…