How do the multiple startups that import YouTube video into their platform work - one's that need the video/audio files? There seems to be a lot of webapps supp

How do the multiple startups that import YouTube video into their platform work - one's that need the video/audio files? There seems to be a lot of webapps supporting this but I always wondered if they have a goto API for doing this. I myself have used microlink at some points for automated video source extraction. https://microlink.io https://microlink.io