How to fit 7.4 billion people in one city

I really enjoyed watching this video explaining how 7.4 billion people (roughly the population of the entire earth) could hypothetically survive in one city. The video first describes a bunch of densely populated places that either exist or existed (looking at you Kowloon Walled City!) in the history of the world and then proceeds to imagine what it would be like for the entire population to live in those city limits. Doesn't sound like fun. Crazy to think though that places like this exist where you literally can't turn a corner and not see a human face.