A headcanon is more or less someone's personal interpretation of a piece of media's lore, when there's no text explicitly confirming it
some headcanons i have:
Those giant space stations have a lot more rooms, and may also serve as a inn for visiting pilots, among other things.
There are plenty of planets with massive sprawling cities, but either our traveler just prefers to explore the sparse worlds, or they're well hidden to avoid the sentinels
The reason why we get units for 'discovering' planets that are already occupied by a species is because it's new to the traveler's race
Thinking about it more, I think it’s maybe even bleaker. Forget the galaxy hopping for a moment. We have two modes of travel, portaling and faster than light warping. Lets say portaling is some sort of instant quantum entanglement something, but faster than light travel is going to have relativistic time dilation effects. Because we can still find the same people after a warp, everyone must be functionally immortal. Otherwise we’d have never see the same NPC twice, if we traveled by starship.