Can a tag launch a podcast episode?
A show, yes, and that is the version worth building. A specific episode is possible and almost never useful, because the episode you want is the newest one and the tag holds whichever address you wrote months ago. A tag pointing at a show follows the show.
Point at the show
A show has one address that stays correct as episodes appear. Write that and the tag stays useful indefinitely.
An episode has an address that was correct once. Write that and the tag becomes a bookmark to something you have already heard.
This distinction is the whole of the decision, and it is why most podcast tag projects that get abandoned were built on episodes.
When an episode makes sense
Two cases, both physical. A printed thing that refers to one recording, such as a card in a book or a note beside an exhibit, where the episode is the point.
And a gift, where you are handing somebody one specific thing to listen to rather than a subscription.
In both, the tag is attached to an object that is itself about that episode, so the tag and the object go stale together, which is the honest arrangement.
The application decides
Whether tapping opens your podcast application or a web page depends on the address you wrote and on what the phone has installed.
An address belonging to a particular service will open that service, which is fine on your own phone and unhelpful on somebody else's.
Test on a phone that does not have your usual application installed, since that is what a recipient's phone looks like.
Take NFC Tools. On iOS it requires an iPhone 7 or later running iOS 15.6 or later, and the app must be launched manually. Background reading is available from the iPhone XS and XR onwards.
Where to put it
Where listening starts. By the running shoes, in the car, on the machine at the gym, beside the ironing board.
The tap has to be part of a gesture you already make, because a tag that requires a detour is a tag you use twice.
And keep it unlocked. Tastes in podcasts change faster than almost anything else you would put on a tag.
Frequently asked questions
Show or episode?
Show, in nearly every case. An episode address is a bookmark to something you have already heard.
Will it open my podcast application?
It depends on the address and on what is installed. Test on a phone without your usual application before giving it away.
Can the tag play the newest episode?
Only in the sense that the show's own page shows the newest one. The tag holds no logic of its own.
Where should the tag go?
Where listening already starts. A tag that needs a detour gets used twice and then forgotten.