Source GM Core pg. 146
From frontier encampments to interstellar space stations, settlements play an important role in shaping the cultures of your setting and the themes of your campaign. Developing a setting can be a daunting task, but these guidelines help give you a start in designing your campaign's next port of call!
In Starfinder, settlements are where characters can rest, recharge, retrain, and dedicate themselves to other downtime activities, all in relative peace. Traditionally, an adventuring band comes together in some kind of settlement, be it a small colony recently established on a moon in the Vast or a bustling continent-spanning city on the surface of a densely populated world. Some adventures take place entirely within a single settlement, while in others the party visits settlements only briefly between their adventures in space.
The first thing you should consider when building a settlement is its role in your story. Is this a major metropolis the heroes will visit again and again during their adventures? A backwater outpost where their adventures begin? The distant interstellar capital from which an evil tyrant issues cruel edicts? The settlement's campaign role will inform many of the other decisions you make about the place.
Once you know why you need the settlement, consider why it would exist in the setting. While established settlements certainly need access to sundries and supplies, local access to these resources are only necessary in low-tech or isolated outposts. Metropolises can even exist on worlds devoid of any naturally occurring food or water, although they're likely populated largely by creatures who don't need to eat or drink. These settlements need to be in such a valuable location that it offsets their need to import necessary supplies. While it might be easier to create a space station or city merely to serve the characters' needs, determining what function it has independent of the characters adds verisimilitude and can provide hooks for further stories.
Settlement stat blocks describe the components and traits of a settlement. You don't need to create a settlement stat block, but doing so can help flesh out the community you're building, and they can be a useful reference for your game.
Source GM Core pg. 147
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. 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.
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.