A sphere is the ideal shape to distribute the force of the atmosphere inside the ship on the hull. A cube would not distribute the forces evenly and would need to be re-enforced in places.

Since the asteroids are silly, and the skateboard appears/disappears, and the targ just appears, I was thinking this was some kids playing in a holodeck, maybe an in-universe hologame(?) for kids.

I certainly wasn’t expecting it to take place after Discovery, I thought it would be around the same time as Prodigy. I guess restarting the academy after The Burn could be more interesting, either way, I’m looking forward to seeing it.