Cosine similarity between two folders (1 and 2) with documents, and find the most relevant set of documents (in folder 2) for each doc (in folder 2)
Similarity is computed in a vector space model with a primary assumption that the pair of objects for which you want to compute similarity must be represented in the same vector space using some pre-processing operation.

Second dataset is a JSON. You'll need to extract text from it. One way to do it: you can use title and tags strings for each video, concatenate it to a string to find string representation for each video. Use any text vectorization method post that.

So get the video metadata into a text and then you can compute similarity.