It depends on what the video is on now. If it's on a camera, it depends on what capabilities for transferring videos the camera has. If it's on a computer, just upload it to Youtube from there.
Transferring videos between phones and computers is easy, Either connect thm by USB cable, ir with wireless file transfer apps, and transfer the files. Transferring them between a camera ans anything else depends on the capabilities of the camera. There gave been so many standards over the years that even if they use the same cable, there's no guarantee that you can transfer from one to the other.