@zip

Yes, Apple Podcasts requires an RSS feed to register your podcast, and this is the platform for most podcasts to then propagate throughout the "podiverse". And Spotify's new hosting platform creates an RSS feed (but doesn't immediately make it public, that's up to the podcast owner - Spotify would much rather you didn't host your podcast anywhere else).

But Spotify and others also offer exclusive shows (i.e. not podcasts). At least they did. And some podcasts don't disclose their feed so other services can pick it up. Some shows I've seen only publish an audio file on a web page or exclusively on Soundcloud for example.