First I have to say: respect.
I think it's great that you are highly motivated at such a young age and even willing to write in many genres.
With that out of the way. I have to disagree about: all humans are mostly the same.
This sort of thing leads to writing a Roman emperor as you would write a modern female radio host. This is the Hollywood approach and a lot of the time it leads to hilarious characters voicing absurd ideas.
I don't want to get into how humans are not all the same. I will touch on that later. But basically people are different.
Different how? Easy. Their emotions, expectation, reasoning...etc.
Sure you can understand bad things in history and even serial killers, genocide, rape, torture...etc. But that does it mean that all humans are equal.
Merely that we posses the intelligence to understand and sympathize with all human actions.
Anyway not to bore you with theory I just do it like this
Mechanical approach. With external context and internal context.
External is where are they? What is their class? What is their family? What are the past important experiences that changed them?...etc.
All of this is solved by deciding the setting.
If I write a story about an average middle class guy in Italy who works as a programmer he is probably Christian, probably loves football...etc. Does that mean all Italians are like that? Of course not. Just the average.
A liberal activist radio host in NY. Atheist, Karl Marx, liberal in certain things, feminist, vegan...etc.
This is not to stereotype or anything. But basically how to represent the average person of the group honestly but have your own twist.
I also despise the approach of writing all characters to be a mouth piece for my and my views. Which is very popular now.
So. Before I lose you here is how what I wrote changes things.
The Italian guy might lament the control of Northern Italian football clubs and how his club will never win. He missed church one day and his mother called him to shout at him and he feels guilty. When he arrived to work to find a huge security breach because an idiot had a password of 123456789Ab.
All those little details adds authenticity to the story, Those are the little touches that when an Italian reads is like: Yep. That's how it works.
Now remove all those things and their effect and you have a grey blob of a human
So. you are always looking for those little touches to add. Like salting your food.
So. I think it is now clear that those external factors play a huge part.
A guy who a millionaire probably won't bother with checking to see if that restaurant is within his price range or not. Those moments of anxiety as you are worried that your card might get declined, despite knowing for a fact that you have enough and double checking you whole order and doing the math, and that invaluable relief when it is accepted.
This can be a point of tension in a relationship.
So. We are now connecting things. Because A is much richer B is finding A's life style problematic. Same with religion or looks or life style...etc.
The realm of absolute mystery and terra incognita.
Well. Not that much.
I can't even begin to mention the big five personality module.
It really helped my in my own life in general and also writing.
So. Even if you are not a writer you need to know about it. It would provide incredible insights into who you are.
I will simply say something like this: introvert and extrovert.
You always want to go out, you always want to stay at home. A very common point of tension.
This is an actual problem in relationships between humans. When A is high and B is low and they are not aware of such a thing it leads to that...etc
How many parent tortured their kids for being introverted or disagreeable...etc?
Same with the rest of the big five traits. If people are not aware of those differences this leads them to believe that the other person is wrong and they need to change. Which obviously causes trouble.
We are not done yet. The internal context is the exclusive domain of the writer.
Here you can play around the most. You can create the most interesting interactions and have a character be hit with tragedy become stronger or weaker.
Here people are changed, made, altered, tested, destroyed...etc.
Because there is not a 1:1 mathematical certainty between experience and mentality here you can do what you want as long as it makes sense.
I just read yesterday this comment on a YT video
I read a story where “that guy” agreed to design his next character
with a friend (they both alternated picking traits for each other’s
characters) He ended up playing a Pocahontas like barbarian whose
tribe was murdered by some guy Obviously his goal was to hunt down the
guy, but his friend said the character wouldn’t kill him Over time
this character developed from stand-offish to genuinely nice and kind,
and at the end of the campaign there was a massive naval battle The
guy who killed Pocahontas’s tribe was leading the enemy navy, and she
walked through a hail of black powder shot and sword blows, dying only
a few feet in front of him The DM allowed Pocahontas one more action
so the formerly antisocial player described his character as she
reached down, beaten and bloodied beyond recognition From a pocket
she pulled out, not a knife or a gun, but a flower from her tribe, the
tribe slaughtered by the man who stood before her She pushed it up to
his chest and whispered “I forgive you” before dying Since then the
player has been unable to play antisocial PCs The actual story is
longer and better, but the idea of players changing by playing
characters they normally don’t seems interesting at the very least
For context that guy refers to board game players, like D&D, who are really problematic from small things to basically forcing you to ban them or even not being friends with them.
I found this comment fascinating for obvious reasons.
So. Such a comment clearly shows how people. If all abused children grow up to be abusive we would all be abused. It is your job and privilege to handle those aspects. Perhaps the lone wolf paranoid ranger is a good person deep down. Perhaps the charitable princess is a power hungry manipulator. Maybe the priest loses their faith once the demon shows up in the room. Maybe the evil drug lord can find redemption.
All those plots are not only about external things but how you decide that the mind and soul of your characters act.
Perhaps I'm too much of a mechanical writer, I guess you can say architect, but honestly this just make a lot more sense to me.
Humans are complicated and solving the problems using actual proven tactics and science seems to help me figure out the larger context of people.
My female character seems to be her our person, or I hope so, because I made sure
that her background influenced her as well as her own basic psychological model
Now call it good or bad this is how I approach humanity. People are usually a lot less complicated if you actually invest the time in actually observing and listening instead of judging.