Bruh
I guess most of the stuff is loaded during the initial load and when jumping between systems (which can be quite a long loading time sometimes, even on PC).
Then the planets are just generated with math, thus it's easier to compute terrain on defined coordinates (nothing to load really, just use CPU and GPU power to generate geometry on the fly).
I guess textures, models, sounds and buildings are streamed and a lower quality version could be loaded during the initial load (you can usually see low terrain quality textures being loaded when traveling too fast, even on foot and when the terrain LOD is the best one).
It's impressive yes, but not that much in the end, planets are mostly limited to one biome each, you can see every single type of trees or rocks on your landing spot and almost never have to go elsewhere on the planet to see all the stuff.
I love the game, don't get me wrong.
And don't compare it to Starfield for example, there's almost no NPC, absolutely no object persistence, they don't have the same scope, and Starfield uses defined tiles, not true procedural generation on the fly.
It could be maybe compared to Elite Dangerous, but in Elite they are going to a more realistic universe, less variety on planets, more empty planets (but true atmospheric planets are still not landable), but as in NMS you can fly from planet to planet and into stations without a single loading time as it's fully procedurally generated.
One thing I miss in NMS is moving planets, I love that in Elite (and Starfield), seeing planets moving across the sky and casting shadows if occluding the sun (at least in Elite), having the planets and stations not always in the same spot in the systems in Elite.