[judge2020]

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

[enriquto]

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

[judge2020]

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

[enriquto]

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