Answer 1

comma operator evaluates left expressions, discards results and returns last expression

printf((printf("2"), printf("3"), "4"));
prints "234"

https://en.cppreference.com/w/cpp/language/operator_other

Answer 2

Because that's the last number among these. What are you trying to achieve here ?

Answer 3

the bracket operator has higher precedence than assignment operator.
the expression within bracket operator is evaluated from left to right but it is always the result of the last expression which gets assigned.