If the user agent is a podcast app (should probably have some standardized UA for this), then you can just return the RSS in a 301 redirect.
This is how firebase app deeplinks work.
Ideally, opening a link for another podcast from an episode's show notes would have access to document.referrer. With that, you could redirect the user back into their app of choice.
Unfortunately, document.referrer doesn't seem to work from app-to-web, just web-to-web. User-agent is similarly unhelpful.
My best solution is to give the user a page with all the relevant links, and then let them save their preference with a cookie.
so if you want to make a QR code that works for everyone, maybe you could use Apple’s feed-based URL scheme and build a QR scanner into each podcast app that intercepts it. So iOS users that just use their a generic QR scanner get Apple Podcasts, and people scanning from their podcast app get a better experience.
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