I haven’t played NMS, but watched a lot of videos regarding simulating planets, atmosphere (and transitioning to-from space) around the time it was hyped, and assumed that’s what NMS is doing. Which is (maybe? I haven’t played) why you can walk around the whole planet, and take off and turn around and see that same planet from space without loading, etc.

NMS as I understand it is a simulation first, sandbox second.

Starfield sounds like Spacerim if anything, with instanced planets that are separate from space. At ground level, planets are just flat planes and you only explore a small, generated chunk at one loaded instance. It’s not actually a spherical planet when on the ground.