VP9 is the default codec of YouTube, but it is not commonly used in video editing tools.
If you need to edit some video that is encoded with VP9 codec, you may need to convert it into some commonly used codec like H264.

The transcoding can be done easily with ffmpeg, by using the following command you may losslessly convert a VP9 video into H264.

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ffmpeg -i video.webm -y -vcodec libx264 -qp 0 -pix_fmt yuv420p -acodec copy video.mp4