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