[mononcqc]

The way I see it, arrays and binaries start a 0 because they represent an offset. Others start at 1 because they represent a position (first, second, third, ...) instead.

For the function and argument orders, there's no explanation for that one.

Erlang strings are a whole other subject, for which I recommend you give an eye to this blog post for the rationale behind a lot of their behavior: https://medium.com/functional-erlang/7588daad8f05 . It's a decent read on the topic.