Where do podcast apps source their data from?

iTunes is still the canonical place to post a podcast and get it picked up by nearly every app.

https://developer.apple.com/library/archive/documentation/Au...

You can either use the iTunes API, or you can use the Podcast Index, which is free and open for anyone. podcastindex.org also has a number of APIs to help build a tool.

The podcast hosting companies (iheart, Libsyn etc)

Apple podcasts, fyyd, podcast index, ...

RSS