The departments that need to be audited for the OHSMS will depend on the scope you have chosen for the management system. With ISO 45001 it is up to the organization to determine which processes will become part of the OHSMS, and this will determine what needs to be audited. I like to think of the scope as “where the PH&S rules will apply”.
Once you know which processes are included, then you will prepare to assess each process against the planned arrangement. In effect, you will be collecting evidence to show that process meets their requirements. Where they do not, you will raise a nonconformity which will require corrective action to improve the system and make the process meet requirements once again.
You can find out more on how scope and internal audit work in the articles: