This is how RTB works, step by step

Real Time Bidding by Fidelity Media
OpenRTB engine at glance
Based on latest OpenRTB and Native Dynamic API fxmSSP platform makes buying and selling of ad inventory more transparent and effective. Real Time Bidding also delivers much higher fill and revenue for publishers thanks to the pure auction system where ad impressions are bought in real-time with the highest available CPM. It takes a milliseconds to choose the best bid among the others and deliver the winners' ad to the visitor's eyes, collect data for reporting and ongoing optimisation.