b.) It's obvious the ads on Spotify aren't the same ads that are in the normal RSS-feed version of the same podcast, which feels very skeevy to me. I have no idea of the business arrangement but it doesn't feel like I'm supporting the podcaster when I listen to them over spotify.
Upon receiving the HTTP request from a podcast client, render a new version of the MP3 file with an up-to-date advert inserted. This version can be cached for a pre-specified period and then replaced later. You can't really get any tracking information to do dynamically targeted ads from the request, but for a podcast this doesn't matter much, as the hosts are typically aware enough of their listener demographic.
Dynamic Ad Insertion. The file server guesses your location/demographic based on the user agent and ip address and then splices ads into the mp3 as it is sent to you.
a.) It doesn't have all the podcasts I listen to
b.) It's obvious the ads on Spotify aren't the same ads that are in the normal RSS-feed version of the same podcast, which feels very skeevy to me. I have no idea of the business arrangement but it doesn't feel like I'm supporting the podcaster when I listen to them over spotify.