I hate videos where the title is a question but could just be the answer, so I searched the answer instead of clicking: it’s Germany’s country code, .de, which has 16.1 million registered domains. The next two are .uk and .cn, which belong to the United Kingdom and China, with 10.6 million and 9 million domains, respectively.
If you had watched the video instead you would know that this isn’t really the point of it.
That’s one way to look at it. Another way is to see it as entertainment trying to get you to watch, not a lecture trying to be concise.
Also, the question in the title has an answer which I think is far more interesting than the one given in the comment a few levels above this, and that is the answer the video gives. Sometimes the story told on the way to giving an answer can be more interesting than the actual answer, and this video, as a bonus, goes through the basics of DNS in a way that is digestible for a casual viewer. In my opinion, these are all more interesting than a guy writing “it’s .de”, and are all valid reasons for the video to be titles as it is.
You’ve got the wrong mindset going into it. Would you really go to a standup comedy and then complain you learned nothing useful? These videos are for entertainment foremost, lecturing second, concise factual information not at all.