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.