Many modern podcasts don't actually include any ads in their source audio, relying instead on dynamic ad insertion.
Many podcast hosts provide dynamic ad insertion functionality. Unlike baked-in ads, podcast creators mark the locations of where ads can be inserted and, when requested by a user, the host generates a new MP3 on-the-fly using the user's tracking data (geolocation, demographics, etc.) that is then downloaded to their device.
This allows old podcasts to continue to generate ad revenue by dynamically inserting ads from currently-running ad campaigns.
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