There's a thing called "server side ad injection" (more like cancer injection) that inserts arbitrary ads directly in the audio stream so the ad is independent from the original producer. Presumably they do some rudimentary tracking based on IPs, user-agent and whether your podcast client downloaded the file past the point where the ad is injected.
I worked in that industry 12 years ago, and already back then indeed there was a simple method to link some ID (usually cookie) to the injected content.
The startup I worked at used it for contests, out of 100k downloads of a podcast, 1 of them would get a different starting or ending segment to give away something. So it was persistent to your user-login on the podcasting portal (which was a thing back then :-))
I'm fine with podcast ads because I want the excellent podcasts I enjoy to continue existing. That said, I cannot stand injected ads, since they're often injected in awkward places and are jarring to the experience. I'll (not exactly happily but not angrily) listen to a live read where the podcaster has at least taken the time to try not to destroy the flow of the entire podcast.
All that said, the best ads (from a listener experience POV) are the extremely short, to the point ads that are honestly too short to put the effort into skipping. Planet Money (and the NPR podcasts I listen to in general) does this well. I'm curious what their CPMs are relative to a typical ad.
