Enders Game instilled the importance of adjusting your frame of reference. What was up can become down.
So it’s science fiction but when you consider a collaborative global human response to existential danger it’s fantasy.
But the Shadow series shows how quickly we go back to weaponizing and using gifted war trained children as tools of conquest… so realism/horror?
It’s like how the earth bulges out at the equator.
The momentum pulls them out, gravity pulls it back. Similar thing at the solar and galactic planes.
They “bulge” relatively to the spin of the material over time and the clumps of material then forms planets along that general plane.
From the aboves video you can see this at 44seconds
https://www.youtube.com/watch?v=sCkhEu3lYNc&t=44s
And a relevant PBS Spacetime video
Why is the Earth Round and the Milky Way Flat? | Space Time | PBS Digital Studios
But the biggest reason is the way gasses form in space. As noted in the above video at the 8:45 mark. Gasses aren’t able to fall back into the center along their axis of spin because of their momentum. But the gassed above and below the plane can move towards the gathering material there by gravity and so it flattens out.
https://youtu.be/Aj6Kc1mvsdo?t=8m45s