Whether you can use someone else’s YouTube video depends on the licensing terms set by the copyright holder, usually outlined in YouTube Terms of Service.
In general, you can reproduce, distribute, modify, and display videos using the provided options. However, users can restrict these actions if they don’t want others to reproduce their videos. In such cases, you can embed the video, as long as you don’t reproduce it, directing users to the original source.
Some copyright holders may also publish their videos under a Creative Commons license, allowing sharing and reproduction under its specific terms.