The videos call returns an array of video that have been added to the media item. If you take an example response:

https://api.themoviedb.org/3/movie/550/videos?api_key=###

You can iterate over the results array and build the YouTube URL by using the key field. An example of the first item in the response above has the key of qvCogW-N-HE so you can then build a standard YouTube URL:

https://www.youtube.com/watch?v=qvCogW-N-HE