>discover that there’s a bunch of exceptions?

Periodically the downloaded videos become unavailable offline if you don't have an internet connection to refresh them. Maybe once a month or something. Which means you can't hold on to a video through their service indefinitely.

Wait, how does that even work?

The downloaded videos likely have time constraint applied to them that can be updated when the app connects to the Internet. I don't think the downloaded videos can be played outside of the app.

Oh, it’s app only? That would already be a pain. I think they got into a good groove of saving URLs for later downloading. Needing to load each one on a tablet is ruining the automation of them visiting a friend or sitting in a cafe, socializing and downing a few dozen gb via youtube-dl just hitting a list of URLs in the bg.

There are no exceptions except for things like paid TV shows (not clips, actual TV shows and movies you can pay for[0]) - you can download any video and, as said in the sibling comment, it only expires after 30 days of being offline.

0: https://support.google.com/youtube/answer/9679194?hl=en

I don't understand the concept of "expiration" for an actual file that you have downloaded. How does it work, technically?

The download expires in the app. You don’t get a download button on YouTube.com with premium.

So the app deletes the file after expiration. But what if you copy the file elsewhere?