I know YouTube is a terrible provider of pirated content and also that it is almost impossible to pirate without a VPN, but I would like to know: if I download a movie from YouTube (directly from it, of course) without a VPN, will I receive “that type of message” from my ISP?
No to be pedantic, but this isn’t true. If you’re watching via the YouTube app, the content is served via chunks, and not in one continuous stream. I’m sure it would depend on the ISP, but they could potentially be able to differentiate the two.
yt-dlp streams those chunks into the final file tbh so it should be fine.