>>2110965
Each territory is constantly assimilating pops to your culture, converting them to your state religion. Growing one random pop in the territory, receiving an immigrant from another territory, and sending an emigrant to somewhere else. Promoting a pop from slave to freeman, freeman to citizen, citizen to noble, or demoting a pop in the opposite. There's also tribesman but they don't want to live in the cities.

Only pops of your main culture, or integrated cultures, will provide levies. You can also interact with other cultures, granting privileges to make them happier, or enslaving them or trying to breed them out.