y=80+(20*(x-1)^{1.15})+((x-1)*0.85)^{2.03}

'x' is the number of districts in the city. 'y' is the cost of the district. I wonder how the formula came out.

I've been playing the game since its launch. However, I am still not sure how should I build cities.

How many Maker's Quarters should I build? Should I build the common's quarter in the middle of districts or maximize the adjacency bonus?

Every single district increases the cost of the next district exponentially. Potential output from adjacency bonuses fluctuates by infrastructures and the terrains that prevent the construction of the districts(mountains, lakes).

I tried to find the optimized strategy but I gave up. My brain's not a computer and there are too many variables.

So now I just build emblematic quarters, harbors, artisan's quarter, and spam military units, conquer neighbors, and attatch their territories with districts they built for me to my city. Some hamlets are more than enough to build afterward. I think it's way more effective than building districts by myself. Even if it isn't, at least it's less stressful, free from all those expense-revenue calculations.

But still, I do wanna find good strategies for city building on the other hand. Could I take some advice from other Humankind players? What's your strategy?

I don't think there's a way to build cities and I'd suggest to go with whatever you like, even if it is not efficient, if it gives you more fun. Just lower the difficulty settings if it isn't working well enough.

That said, I can give you some information about my habits:

1. I tend to favor small cities of 2 to 4 territories, bigger ones being notably too much impaired by the district scaling cost.
2. I get over the city cap by one without thinking about it (I wrote about it here and wished for a way to help playing tall here, but nothing is done yet) and generally stay two above, even three for small periods of time
3. I don't create many districts early on, trying to get the most influence possible (the first town gets generally a MQ or FQ, depending on the yields, but then I rarely build any early)
4. I buy what I can through influence before attaching, getting fishing early when playing on water maps, especially with the Phoenicians
5. I attach only what's needed to get the stars or if I fear a neighbour is coming to get the place
6. I like to create food heavy outposts as city centers
7. but create 2/18 outposts to attached without remorse, they give an early boost to production that is palpable
8. I almost always construct EQ, partly because I tend to focus a lot on culture (going Liberty every game)
9. I forgo garrison
10. I try to buy my stability and avoid CQ
11. I tend to buy production, then food through trade, then what's left, mostly starting with copper and horses
12. When placing CQ, I tend to remedy a lack of stability and the need to get it soon is felt sharper than any long term planning. +10 makes me already happy. I try to plan a bit for later and end up building around them once I make some
13. I focus on some key infrastructures: animal barns and forges do more for a city than many districts with enough trade
14. I tend to place nature reserves, because I like them, despite their lack of interest
15. I the last two eras, I more or less only build science districts (a lot) and key infrastructure.
16. hamlets tend to be not so good, I do like the 4 slots though

All of that coming from someone who:

1. mostly plays on huge map
2. endless speed
3. 10 players
4. humankind difficulty
5. not a warmonger at all
6. solo play mostly
7. not a great player, efficiency wise
8. I tend to go collectivism, world, liberty, progress all my games
9. I tend to avoid focusing on production, finding it a bit meh (I don't play Egypt, Khmer, and so on) and even made a no MQ run once (won with 28K, having however used the Carthaginians EQ)
10. I like to keep pollution down
11. I don't like the „full Coruscant“
12. Fun before fame

First era:

1. focus on food -> it gives influence and can help fill the population slots in cities, giving more than it seems
2. then influence for land grabbing, helpful civics
3. then production (one or two production heavy outposts outweight creating MQ)
4. then gold
5. then science, as I need time to get the stars and don't want to get stuck with extra wasted science

Middle game:

1. Mostly adaptative
2. Production might come in as the default choice when there's a tie
3. Money runs can work
4. Mixed runs, not building MQ but bumping workers through infrastructure can be enough against the AI.

Late Game:

1. Science is all you need
2. Investing in production seems no longer worth it

It's a bit of a micmac response, I hope it can still give you some light on my take.
I'm pretty sure more seasoned and efficient players will pinpoint the flaws in my ways.