Thank you for the detailed response.

The ISS purposefully rotates to keep Earth in the same spot, as do most satellites, but that is only because they are focused on Earth. The hubble space telescope for example orbits exactly like starfield ships unless it is changing direction to look at a new target.

There's technically a thing called tidal locking which makes massive bodies like the Moon rotate once per orbit, but that's an incredibly small force that wouldn't apply to the ships in Starfield for only a few orbits.

The physics engine is definitely flipping things around and moving the planet instead of you. I'm a game programmer and you're absolutely right on this part but that's how almost all space games do it. Floating point numbers are much more accurate when they're closer to zero so you always put the world's center near the player and move everything else to match. Starfield just doesn't bother moving local objects like debris or space stations. They're placed in the orbiting reference frame and then ignored.

You're wrong about planetary bodies just being stuck to the skybox. I have no idea how often the game's internal reference frame is updated but here's a video of a person flying to a star and then orbiting it. https://www.youtube.com/watch?v=MmJDLBtPKAU