It's important to have some idea of a settlement's culture established. While every nuanced detail of its governing laws and cultural norms can often be hand-waved, you'll want a minimal description of the core concept of your settlement, including its style of government and core cultural edicts and anathema. The amount of additional detail you provide depends on the needs of your story. If you have multiple governing bodies involved in your campaign, especially if your story includes economic or political intrigue, you'll want to establish enough information to create reference sheets for your players to serve as helpful reminders of all the relevant factions in your campaign. That said, many settlements in a multi-planetary campaign function fairly independent of any larger governing body, their autonomy assured strictly by the extensive internal bureaucracy necessary to maintain a vibrant multicultural metropolis of millions.
Beyond those basic details, the following considerations can help flesh out the settlements in your setting.
Major planetary geographical boundaries, such as mountains, seas, and large rivers, often present natural borders for a region. Depending on its leadership, culture, and the resources available, a settlement might be as small as an arcology floating over a gas giant or as large as a planet-spanning megaplex. In Starfinder, most settlements tend to be larger than the historical cities of Lost Golarion and might cover hundreds or thousands of square miles or span an entire planet. Widespread technological and magical innovations give governing entities wider reach, improve city maintenance, and foster cultural mixing. These settlements are often surrounded by greater metropolitan areas, which might or might not be considered part of the settlement itself.
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.
What elements of the settlement's predominant culture stand out? A settlement might have an unusual stance on religion, a specific demographic, distinctive natural features, noteworthy political views, or any number of unique elements that differentiate it from other settlements in the galaxy. These hallmarks can inform your decisions about many other aspects of the settlement.
How did the settlement come to be? Has it stood since before
the Gap, a bastion of stability while the rest of the galaxy changed around it? Perhaps it was built over the ruins of another civilization that was destroyed eons ago by powerful invaders still traveling the distant stars. Or perhaps it's a young settlement, founded recently by refugees seeking to escape the Swarm. Is it the first settlement on a planet of untamed wilderness or a city of refugees built on the same asteroid that ruined a once-thriving metropolitan world? How have the residents of the settlement adapted to change, and in what ways have they failed to do so?
Determine the key resources and industries that drive the settlement's economy. The availability of natural resources, skills and specializations of its citizens, and relations with its neighbors can all be factors in determining a settlement's economic success. For example, an area with few resources might be more reliant on technological innovation or military prowess, while a settlement rich in resources will more likely develop an opulent upper class composed of those with the capital to exploit such resources.
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.