How to Absorb a City in Humankind
As you grow your civilization and build more outposts in Humankind, you'll append more territories to your cities, growing your area of influence and getting more expansionist stars in the process. However, there are only so many settlements you can have at any given time, and you might start wondering how to absorb a city.
Absorbing makes one town part of another, merging the territory of each into a single area. It also lowers the total number of cities you have in an empire, allowing you to build more without increasing the city cap through techs or civics like Leadership.
To get the action, you first need the Military Architecture technology, which becomes available in the Medieval era. This tech also gives you the fort, trebuchet, and one other civilization-specific military unit.
Once you research Military Architecture, you'll see Absorb City action to the right of a city's name on the map. Select the settlement you want to merge into, then find the town you want to absorb and click the action button assuming you have the necessary amount of gold required. Doing so makes it an administrative center for the primary city and adds all of its districts, as well.
You can detach cities from each other whenever you want, as long as the one in question is not a connecting territory. A connecting territory is one that attaches outposts or other settlements to the primary city and are only connected by the merged city. You must detach connected territories first.
It's important to know that merging towns and areas like this greatly affects the stability of the city you're merging into because you will add many districts at once. You'll also must be sure you can handle the increase in population by having enough food and building enough Farmers Quarter districts ahead of time. Enact the Legitimacy civic if you can; it reduces the cost of absorbing cities by 20%.
You can absorb the city of another civilization in a similar manner. First, go to war, occupy a city by laying siege to it and winning, and then acquire a city when forcing surrender. Of course, you could just wipe them off the map or choose an expansionist civ, like the Assyrians, allowing you to use Under One Banner, which forcibly annexes cities and admin centers.