It's just an mp3 file!
edit: I should point out that i pitched, to Apple, the ability to dynamically insert ads sometime around ipod 5th gen, 2005-2006, but not for podcasts, but for downloadable videos, like "last night's TV show". I'm sorry i did this. I don't mean i had the idea and said "hey this is an idea", i had the entire infrastructure documented, it was drop in and go. Whoops.
DAI is a thing and my initial point several comments ago was that too many podcasts implement it, let advertisers drop their stuff in (not their own read), and never check what’s actually being run on their show. That’s all my point was about.