I don't work at YouTube, but it is highly unlikely that they have a Brexit department or a divisiveness department. What they have is machine learning.

The goal of their Algorithm is to maximize for watch time. Any behavior that leads to longer watch time will be promoted.

If a user watches 2 videos, leaves the platform for a while then comes back later to watch a third, the system will have learned what video to suggest to another person who watched only the first two. If the system can't determine what to show next, it will choose the video that has a record of increasing watch time.