The Organization creation rules in the
True20 Experts Handbook are very good and can be used to build large communities for any genre. Its size table defines tiers of membership that can be as large as 660,480 members - why it caps at that #, only the writers know.
You could easily homebrew higher tiers following the existing tier progression.
The organization/communities mirror PCs, in that they have Abilties, Backgrounds, Roles, Feats, Skills and Reputations, as opposed to being a bundle of settlement stats/features rolled up from random tables. The Ability scores and feats are a list of those that are most common among members, and skills and skill ranks are the same. With the idea that were you to build a PC/NPC member for an org, you'd look at those features to establish common baselines. The orgs also reflect the core PC roles in their membersip, in that there's Adept organizations, Expert organizations and Warrior organizations. But there's also Ordinary organizations and Mixed-Role organizations, which are what you'd associate more with a community or colony.
The coolest thing about them though, is that Orgs have Combat, Financial and Social attack and damage values. Those can be used for abstract conflicts in which one organization confronts another. So while say a Warrior Org comprising an army could could use the Combat attack to battle another, an organization that's religious could use the Social attack to prothletize against another, and a multi-national corp could use the Financial attack & damage to pull of a corporate takeover. Needles to say a community could use any of those in all sorts of interesting ways. Those play out similarly to Savage World Mass Battles, if you're familiar with those rules.
Orgs also have a Reputation stat that can be used as a bonus by either it, or a member. And there's a Wealth bonus that reflective of its overall financial worth. There's som e emphasis on Org Leadership in how it corresponds to military, financial and social leaders. As well as how leaderhip can play into recruitment and how it influences organizational conflicts. Meanwhile there's narrative and play mechanics whereby a PC can gain influence, or loose influence with an org.
There's no denying they're a fairly crunchy set of rules clocking in a full 8 pages. But theyr'e flexible and able to support interesting roleplaying and diverse settings. It's easy to adapt them to D&D 3e, PF1e or d20, on account of them sharing common DNA. But certainly possible to adapt them to other TTRPGs.
For community building on another world - IME both Traveler and Star Trek Adventures have good rules for that. Albeit both are fairly high level in the details.
