[NEWSTUBE] Why YouTube’s Video Files Are Not Responsive In Single Video Page

It is because the path of URL of YouTube does not receive the short form, it only receives the full URL, so you should not put the URL having the form as below
https://youtu.be/fQnLmth9f2w

You must use the URL as follows https://www.youtube.com/watch?v=fQnLmth9f2w