Google Ad Manager - Delivery of ad

About

The user's web browser or mobile device loads an Ad Manager ad tag (on a site) or Ad Manager ad code (in an app) and triggers an ad request, which passes information to the ad server.
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.
When Ad Exchange or AdSense is eligible, Ad Manager uses dynamic allocation to check if a remnant line item can be served without affecting the delivery goals or pacing for the candidate line item. This increases overall yield for the network without increasing the risk of underdelivery.
The ad server selects the best creative from within the selected line item or RTB demand.
The ad server serves the creative to the user.