> A = B => not()   and  (A=B) => not()
>
> Are different

Yes, the `=>` operator  has higher precedence than `=`.
>
>
> A/B/C => distinct-values()  and    (A/B/C) =>  distinct-values()

>
> Are the same

Yes, the `/` operator has higher precedence than `=>`.
>
>
> Where can I find the specifics?
>

Appendix A.5 of the XPath spec gives a summary of the operator precedences.
The normative specification is the EBNF grammar.

Michael Kay
Saxonica