prediction:
- main plot locations are more handcrafted plus quests/factions
- the rest of non-plot locations are just fillers to do gathering and fight procedural generated mobs settlements.
- I don't think it's a big galaxy anyway, but probably a couple hundred systems with about 50~70 populated by the developers. So more like "local region".
did a bit google and I am right on the last bit already. source
There are over 100 Starfield star systems to find and explore, grouped by level that create difficulty tiers depending on where you go at your current level.