This. An audit's goal is to determine whether the accounting systems and processes at a company are reliable. This is why auditors will test and document the internal controls of a business in the summer to make sure the controls are working properly. Then the audit rolls around, they can use what's called a "reliance strategy" which is based on the fact that the internal controls are reliable and working. They collect documentation and select a statistically representative sample of transactions, but analyzing the minutiae looking for fraud is not the goal. Auditing is about studying accounting systems.

The accounting firms do fraud examination as well, but that is a wholly separate business than audit.