Organisational annual audit plan and audit of each department.

ISO 9001:2015 is not about departments, but about processes. So, determine which processes cross each department. When auditing a department you will audit the requirements around the processes that cross that department.

For example, purchasing department may be about purchasing (clause 8.4), about quality control at reception and nonconformities (clauses 8.6 and 8.7), about identification and traceability (clause 8.5.2), about production planning (clause 8.5.1), about preservation (clause 8.5.4), about warehouse conditions (clause 7.1.4). If you check, I identified the clauses by following the process since identifying a need for purchasing until receiving and storing in the warehouse.

About the annual audit plan, you should consider auditing all processes and ISO 9001:2015 clauses in two or more internal audits along a year.

The following material will provide you information about internal auditors:

• ISO 9001 – ISO 9001 internal auditor training: Is it for me? - https://advisera.com/9001academy/blog/2015/06/02/iso-9001-internal-auditor-training-is-it-for-me/
• What is the ISO 9001 audit program, and how does it work? - https://advisera.com/9001academy/blog/2017/01/24/what-is-the-iso-9001-audit-program-and-how-does-it-work/
• Free online training ISO 9001:2015 Internal Auditor Course – https://advisera.com/training/iso-9001-internal-auditor-course/
• Book -  ISO Internal Audit: A Plain English Guide - https://advisera.com/books/iso-internal-audit-plain-english-guide/