Which Podcast Distribution services does the built-in Windows 10 Mobile Podcasts App link to?

I often cannot find popular podcasts using the built-in podcasting app. Other times, I can find certain podcasts by using other apps, but never seem to find everything in one place. I would like to contact Microsoft and even the podcasters themselves to increase distribution and/or search options. Unfortunately, I don't really know enough to get started. I would prefer to use the built-in Win 10 Mobile app. Any suggestions would be appreciated.

I think due to the fact that podcasts use the open RSS publishing format, you are probably never going to find a comprehensive single listing. And since Apple hijacked the scene ages ago, you'd probably be best served using one of their lists instead. Luckily most of those links will work.

Or google.