YouTube is a video hosting platform, considered the largest one in the world (see its Wikipedia article).
Most YouTube videos are not eligible for upload to Wikimedia Commons because they are still copyrighted and are only offered under the Standard YouTube License, which is not accepted here because it is not free enough.
However, YouTube offers the option to license videos under the free license Creative Commons Attribution 3.0 (CC BY) which is compatible with Commons. Under Commons policies, these videos may be transferred to Wikimedia Commons, with the usual caution that you should not upload work that appears to be plagiarized or is out of scope. Be also aware of YouTube's terms of service.
Some other videos don't use this license option, but include it:
Uploaded videos should be tagged with {{YoutubeReview}} to request a license review to permanently document that the video really was available under that license. If they are released under CC BY 3.0, you can add the license template {{YouTube CC-BY}}. Additionally, you can use {{From YouTube}} to format the source information.
Search videos licensed as Creative Commons on YouTube by doing a search there. Then a filter menu will show up with one of the choices being "Creative Commons". You can also use Vera's Tool to find all open-licenced videos in a given YouTube channel.
Screenshots are similarly handled. That is, screenshots might be acceptable only if the videos are freely licensed or in the public domain.
For advanced options, see Commons:YouTube files/Downloading.
To change the licence of your own videos on YouTube, so you can use the above tool to copy them to Commons, use this process (valid as of November 2020) on a desktop machine:
You can also change the default licence for new uploads to YouTube:
On the opposite, if you want to reuse a Commons video or image as part of your new YouTube video, you must:
Ways to get help
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