What ISO Standard does ISO 27001 Auditor follow during Audits?

ISO 19011 is the standard used for auditing ISO management systems, including ISO 27001. You can find this standard here: https://www.iso.org/standard/70017.html

For certification audits the ISO 27006 needs to be taken into account. This standard specifies requirements and provides guidance for bodies providing audit and certification of an information security management system (ISMS).

For further information, see:
- How to perform an internal audit using ISO 19011 (PDF) https://info.advisera.com/free-download/how-to-perform-an-internal-audit-using-iso-19011

These materials will also help you regarding audits:
- ISO Internal Audit: A Plain English Guide https://advisera.com/books/iso-internal-audit-plain-english-guide/
- ISO 27001:2013 Internal Auditor Course https://advisera.com/training/iso-27001-internal-auditor-course/