RSS feeds are XML files which contain a list of documents hosted on the internet (articles, audio/video). The feed entries contain basic metadata (title, date, author, summary) and a link to the original website (or audio/video file in the case of a podcast).

Feed readers send a simple web request to the website hosting the feed, downloading it if it has changed since the last update. The content is then combined with other feeds and displayed. This way you can have a personalized news reading experience without needing to create an account at a a central provider or open every individual site.

Alternative YouTube clients use RSS feeds provided by YouTube (example: https://www.youtube.com/feeds/videos.xml?channel_id=UC2DjFE7Xf11URZqWBigcVOQ), but they are only used to update subscriptions. All other requests (search, watching videos) are handled by the same web interface as the YouTube desktop application. Fetching the RSS feeds is a lot faster than opening the channel page, so the RSS featuee allows you update 100 or more channels in a few seconds.

The way podcast ads work is either just like YouTube sponsorships (the podcaster gets paid by a company to speak an advertisement themselves) or they are dynamically inserted by the podcast provider (these are the interrupting ads). Since most podcast apps dont store cookies, there is no way to track users and personalization is done only via the IP-based location and topic of the podcast. RSS-based podcast players have no way of directly reporting back playback telemetry. The server hosting the podcasts can only count the number of downloads/playbacks. So there is no way to count the amount of watched ads when using a RSS-based podcast player like AntennaPod or Kasts. Note: this does not apply to podcasts on Spotify, Apple Music or similar platforms. These platforms absolutely track your listening activity. I have no idea whether this affects ad/sponsorship earnings.