How do you develop the cultures of your world?

How do you develop the cultures of your world?
What proccess do you use?

>Make map
>Put Nations and City States on map
>Campaign always starts with all adjacent entities in conflict
>Decide what each individual conflict is about
>Decide why each side is fighting for said thing

Usually thats enough to decide that matters to a certain group and it helps define the different peoples in my world.

But sometimes I always write down; the perception of others of this group"
Because i think it can help to say "what do people in the setting thinking of X"
Then you can say, yes they are being presented that way, but maybe there are interesting reasons beyond the aforementioned conflict for why people see them that way, or maybe you subvert it and the public perception is incorrect.

If you wanted to go really far, you could make a matrix of every group, and write down what each group thinks of each other group.
Because it not only helps define what a group is like due to how others think of them, but it also helps define how a certain group thinks.
Maybe group X is super Xenophobic, so their perception of every group in the matrix is negative. Or w/e.

I blatantly steal from existing historical cultures. Not really, though, but I look at cultures that developed in the same kind of environment and try to figure out why it developed that way. Also borrow some of the aesthetics, which is course also very dependent on the environment. Then what doesn't work with this fictional culture, because it's inspired by things that only really exist in our world, will get extracted, new things added that are part of my world, until I get something that I can call "original".

Take a whole bunch if references and real world stuff then make them have sex, abort the baby, and resurrect it with necromancy

Start from the absolute beginning. Develop a realistic (ish) map with believable rivers and mountain ranges and the like. Think about the wind patterns, erosion, et cetera and how that would affect the climate in certain areas.
Once you have that, decide the origin of life in your world. Was it intelligent design, or did it evolve? Then move from there. How do people spread, what groups end up where? Because you have a thought-out world they're living in, you can develop their cultures organically. You won't have Roman-style architecture in some taiga bereft of good building stone. Here's where you can get creative: You'd more realistically have something like Russian or Nordic style wooden architecture, but maybe some fantastical plant grows in this region and the inhabitants make use of it in their architecture. Then from there, how does this affect their culture?
It can be helpful to have several world-building 'phases'. After you've done the geography and initially seeded the cultures throughout the world, decide on some major events that happened that mixes things up. It could be mundane like a famine, or fantastical like the introduction of magic or invasion by demons. Think of how it affects people and what they then do. In our history, the Völkerwanderung (the Wandering of People) is thought to have been (at least in part) caused by the Huns and the cooling of world temperatures, pushing the Germanic tribes directly in to conflict with the Romans and resulting in the fall of the Western Roman Empire and the origin of Germany.
So, in a similar manner, your event pushes groups around and mixes up the order of the world. Perhaps humans are forced from their frigid northern homes by the growth of the frost giant empire or whatever and flood into Elven lands. Then think of the implications, perhaps the Elves have a revanchist streak and want to take back their land? Maybe the humans forced to move integrate Elven practices?

A combination of random rolls alongside my own ideas mixed in with stuff the players interprete when they're cool ideas.

>desert colony on a mining world
>water is the most expensive thing a local will buy weekly
>terms like "worth his '60" replace "worth his salt"
>water worship is also seen as stupid, its something you need to survive, not a deity
>tampering with aquefiers gets you a death sentence and all bodily moisture industrially sucked up for use

That is one of the things I often find to be lacking in sub-par fantasy. The world is 'one-phase', all the cultures/races were plopped down where they are now, and there's no change in their history.
It can be cool to have some places where X race has been in place since time immemorial, but also have places that have changed hands many times. Humans building a civilization on top of the ruins of an Elven kingdom, or some 'raider' culture conquering a rich country and becoming its rulers, for example. These sorts of events could spice up a world where 'this is kingdom XYZ and has always been kingdom XYZ, this is where orcs live and has always been where orcs live, et cetera.'

Additionally, it's critical to think of what materials are available. What plants, animals, crops, building materials, etc? A culture is almost always a product of what is available to them.

Finally, make sure you're not making even group monolithic. This is one thing that really triggers my fucking autism. Having all Elves be like other Elves and all Orcs being like other Orcs is not very interesting. There should be a few racial traits they all share, but there shouldn't be a 'Elven culture'.
This is something I appreciate about Tolkien's Elves. The different clans are all at least somewhat different. The wood Elves aren't a whole different race, they're just a different culture/ethnicity of Elves than those in say, Rivendell, because they've been shaped by their environment.

>take existing culture
>exaggerate a feature
>mix in two other culture features
>smooth the whole thing over

This seems more like politics generation than culture generation, but it is decent for that
I feel as though this is a necessity to a degree, but it's really dangerous and you might end up with fantasy Japan and fantasy Germany or whatever. You have to be deliberate in your goal of creating novelty, which I think is a better goal than originality, though they are similar. I think instead of starting from life you should start from nothing and draw in inspiritions from life.
I tend to agree, too many world builders build to just fill in their map. I think people need to focus more on the history and adaptation of their peoples to create intresting cultures.
As for races WAY too much do people use them merely as an analogue for human ethnicities. Races should depart from humans in a significant inherent way so that their various cultures take of a different character than the settings humans, either because they have different needs, different life lengths, different ways or reproduction, or different magic or bodies. Something to make them "not human" other than pointy ears and wearing clothes from their monoculture.
I think this has a tendency to create flanderized cultures, because you end up filling in the blanks of minor aspects of culture not with something unique or intresting but with things just lifted from the real culture.

1) Make a generic fantasy map.
2) Drop the Balkans right in the middle of it.
3) Simulate 50-500 years of history.
4) ???
5) "When suddenly, the slavs arrived".