In programmatic advertising, a DSP (the buyer's tool) connects to SSPs and exchanges (the sellers' tools). When an impression becomes available, a real-time auction decides which ad is shown – all automatically, in milliseconds, based on the rules each advertiser set.
This replaces manual deals with software, letting one advertiser reach inventory across thousands of sources, target precisely, and adjust campaigns instantly. It is how most digital display, native and video is bought today.