1) conservation of angular momentum,
2) our universe has three spatial dimensions and the same does not hold in a universe with four spatial dimensions.
I completely agree with the first point, and I think it would be nice to add that under the constraints of conservation of momentum and angular momentum, the total energy is minimized if all particles lie in the same plane. I have doubts about the second point, which the video barely explains anything.
In summary, I have two questions that I need help with.
1) If we can visualize angular momentum in 3D as a vector, how should we think about its counterpart in 4D, a tensor? And what about the law of conservation of angular momentum.
2) What is the shape of the solar system in a universe with four spatial dimensions?