Auditors use their checklist to go into reality, within the audit scope, to collect objective audit evidence with interviews and observations. Auditors interview auditees. What auditees say are not facts, what auditees say are pseudofacts. So, what auditees say must be corroborated with documents or records or direct observation. Auditors basically use documents, records, and direct observation to collect objective evidence.
The following material will provide you with more information:
Free webinar How to perform an ISO internal audit - https://advisera.com/9001academy/webinar/how-to-perform-an-iso-internal-audit-free-webinar-on-demand/
ISO 14001 Internal Auditor Course - https://training.advisera.com/course/iso-14001-internal-auditor-course/
Book - ISO internal audit: A plain English guide: https://advisera.com/books/iso-internal-audit-plain-english-guide/