I’m not very good at this either, but you need to make that stub have exacctly the same stuff that youtube would return. So it needs to have some subset of what wget https://www.youtube.com/embed/wi2jAtpBl0Y returns. That’s what your spec needs as its input.
wc wi2jAtpBl0Y
6 251 65245 wi2jAtpBl0Y
That’s a lot of text to make sense of and stick in your stub.