Google doesn’t seem to care much about enforcing this on an individual basis, especially for casual downloaders, but the company does occasionally play a cat-and-mouse game to try and hamper third-party apps that are designed for this purpose. Personal viewing is generally the only reason to download YouTube videos if you want to avoid ethical issues and copyright violations, and that can be done within the YouTube application with a Youtube Premium subscription. You should never download someone else’s video in order to distribute or earn money from it, whether it’s a video created just for fun or by a content creator making money.
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