For the system audit, you just need to follow the ISO requirement, check whether it fulfill the requirement. For the process audit, it is more details, and you have to go into "how it works".
For example. To audit the system on the purchasing, you need to check whether the input and output are same, all the record is able to provide trace. But for the process, you need to audit how they control on the input and output, not only documentation but the actual practice