I have downloaded a youtube video using the terminal but now I don't know how to play it. I have checked the download folder but I didn't find anything.
So, how can I play this file?
The simplest approach is to just download the file to your current directory and play it from there. This can be done with the following command:
totem "$(youtube-dl youtube.com/watch?v=ES6OyV-RfHU |& grep -oP 'Destination:\s\K.*')"
The command above will both download the video, and extract the file name from the output and automatically launch the default video player to play the file.
Note that the player will not open until the file has finished downloading.