Morality is also subjective.

I agree but see a complex queation being simplified. Unfortunately, I am one of the curmudgeons who see things as not so B & W.

At one time I decided to know the truth regardless of how I felt. I had to be open to all ideas and look at things from a rational and pragmatic perspective. Be careful for what you wish for! Here is a starting example (and it driven my life).
[en.wikipedia.org]

Good answer.

Societal morality arises organically from social interaction and negotiations (explicit and implicit).

I've had some interesting and utterly frustrating conversations with fundamentalists about this. They are super hung-up on the notion that without god deciding what is [im]moral, then everything is just empty, non-binding opinions. They are obsessed -- and I mean OBSESSED -- with the notion that an effective moral code needs a backing authority, a sponsor if you will. Otherwise they just hammer at you endlessly with "that's just your opinoin".

Well no -- the immorality of leaving a baby on the railroad tracks for example is actualy society's opinion, a sort of meme transmitted to each of us through a thousand micro-transactions throughout our lives. Yes it's reinforced by our hypersocial nature and our need to paritipcate in social reciprocity. Yes it's assisted by mirror neurons that allow us to imagine how the baby would feel with the great big train bearing down on it. And by allowing us to imagine how we'll feel later on if we don't save the baby. Even by imagining how saving the baby will increas our social standing and acclaim, potentially.

Empathy and reciprocity, rational self-interest (including long-term), the need to belong. This is what drives morality.

One reason fundamentalists can't accept these facts is that they are conditioned to regard their fellow man as depraved and unredeemed and duplicitious apart from the imagined sanctifying power of salvation. One of the biggest sea changes in my life after deconversion was realizing that my fellow man wasn't worthy of the disdain and suspicion I had reflexively heaped upon hi throughout my life -- and that I was not the paragon of virtue I saw myself as to begin with. But until that happened ... it seemed silly to imagine that humans could self-govern AT ALL, much less far better than my imaginary friend could control or "restrain" them.

While the Trumpocalypse as sorely tested my newfound equanimity about humanity as a whole, I suppose the larger point is that trustworthiness and moral uprightness are a function of the benefits one brings about or the harms one avoids by one's actions and attitudes. It is not a function of how people outwardly adhere to some arbitrary ruleset on penalty of eternal perdition or just divine chastisement.

Within each culture there is a generally acceptee code of whqat constitutes moral behavior. Still, each individual defines for himself/herself what is the basis of his/her moral code and also defines the specific tenets of that code. In actuality, is is more of a personally defined and observed code of conduct.

I don't believe in using he word morality. That's a religious construct. Ethics is another thing.
What is my ethical duty to self, family, community, world. Religion has stained certain words for me and morality is a word often used to assign things as WRONG which are absurbd, harm no one, and are only wrong in religious context. Which is why you see so many people in politics and religion that are "moral" (picking the religious stance on anything) but as unethical as Loki.

Its only some of us and more likely because its a baby. I could not leave a little bird in pain myself or skin a fox alive but many would and many more wouldnt care.