- The ad server gathers information from the ad request.
- The ad server creates a list of all line items matching the targeting criteria.
- The ad server selects the best line item to serve by scanning the list of eligible matching line items, starting with highest priority and working down until it identifies the best candidate line item.
- The ad server serves the creative to the user.
With ad manager, see Google Ad Manager - Delivery tool (Ad Serving Diagnostic)