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If you pay for a youtube subscription, there is a download button right next to the share button.

Correct me if I'm wrong, but isn't that forcing you to use their system when offline and not truly giving you a copy of the mp4 file?

Correct, and I believe you can't even access the video if you the app can't phone home after a certain time.

Also if you're in (or even visiting) some (most?) countries this feature doesn't work. You don't get any refund either.