But what OP refers to is that kids are born with their own personality which already emerges from a very young age in the first year.
I've seen it with my kids, and also other young kids in my close environment. For instance, some are just naturally extraverted, or wild, while others are careful by nature. Some of it changes when they get older, but a lot stays the same. So yeah environment matters, but most parents I talk to agree with me kids come with their own character.
I expect parents expectations of how much they can shape their children has caused more broken adults than anything else.
The extrovert we teach control and restraint, the introvert to be ongoing and more expressive.
Especially parents of identical twins.
In the case of identical twins, their (biological) nature and nurture (upbringing) are the same or very similar, yet their innate character can be very different.
If you believe in the existence of a non-physical soul/spirit, that could be an explanation.
But we don't all have the same home environment. There are generally some years between births, and things happen in those years. Good things, bad things, and things. It'll be more pronounced if there is more time between children.
I mean, just myself: I have two siblings. One is 6 years younger, the other 11 years younger. Between me and the older of the two: My dad graduated college and got a decently stable job. My parents moved away from my mom's family, who helped take care of me while my mother worked, and moved to a small town. My mother started staying at home since the finances allowed it.
We moved yet again before my younger sibling was born: My father switched careers and made more money than before (which was rich compared to their finances while I was young and my father was in college).
All this meant that we got attention from different sorts of folks while young. Our parents had different finances: While I had poverty while young, my youngest sibling had nothing of the sort. My parents were more experienced by the time my brother was born, too. We all had different sorts of challenges which definitely affected the ways my parents communicated with us - their continuing maturity did this as well: I was born before either were 25, but my siblings after.