Fun fact: Zoom doesn't seem to have a maximum video size, so go ahead and add whatever videos you like.
Important: busy backgrounds can be distracting to others. Please be respectful and keep your video background choices subtle.
For copyright reasons, you may have difficulty downloading videos with music in them.
Example: https://www.youtube.com/watch?v=d0tU18Ybcvk
If you have any specific start or stop times, note them.
There are a wealth of YouTube download sites on the Internet that let you to transcode YouTube videos into other formats. Choose any one you like, but be sure that it supports mp4 format.
I like https://www.clipconverter.cc/ because it supports start and stop timestamps.
Use your transcoder site of choice to convert your video, and download the resulting mp4, ideally to a dedicated directory for Zoom wallpapers.
Be sure to choose a meaningful name so you can find it later!
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Zoom -> Preferences -> Virtual Background
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Click the [+] and select "Add Video"
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Select your newly downloaded video.
Your new video is now in your backgrounds library. Enjoy!