Detailing Settlements
Beyond those basic details, the following considerations can help flesh out the settlements in your setting.
Location, Size, and Population
Populations ebb and flow due to a multitude of external factors. Advances in sanitation, medicine, and agriculture can spur dramatic population growth, while war, economic downturn, or plague can devastate it. A megacity has at least 10 million residents. Megaplexes formed of multiple megacities might have populations averaging 50 million or more. A multi-planetary alliance might have billions or more.
Population size is only part of the equation. Figuring out the ancestry ratios of that population and brainstorming how the members of various ancestries interact can often lead to interesting story ideas, or at least give you some jumping-off points when dreaming up how the settlement was founded and its later history.
Cultural Hallmarks
History
Economy and Political Stances
These resources can also affect political relationships. An area poor in a specific resource might have a strong trade relationship with another world that has it, or they might rely on conquest and war to plunder what they need! Settlements also disagree about political structures, public policy, religion, and any number of other factors.
You'll also want to consider the significant NPCs of each settlement. This includes the official rulers, but it also includes other major players, whether they act in an official capacity or entirely behind the scenes.