Does YouTube allow massive scraping like this in their ToS?
Per HiQ vs. LinkedIn, it doesn't matter what their ToS says if the scraper didn't have to agree to the ToS to scrape the data. YouTube will serve videos to someone who isn't logged in. So if you've never agreed to YouTube's ToS, you can scrape the videos. If YT forced everyone to log in before they could watch a video, then anyone who wants to scrape videos would have had to agree to the ToS at some point.
It won't serve me videos if I'm not logged in. It tells me to sign in to prove I'm not a bot. How do these people get around this?