Should the marker be responsible for explaining assignments and exams or the instructor?
That depends very much on the type of questions.

Normally a marker who is not the course supervisor / instructor should be given a sufficiently detailed marking schedule. However, even for a very detailed  schedule there often is still room for a marker to make judgement calls, and the marker should clearly apply these as consistently as possible. When students simply want to understand why they got the marks they got, then that question should be answered by the marker, in particular when such judgement calls are involved. When a student does not agree, this should be escalated to the instructor.

Other questions, for example whether the marking schedule itself is correct, or whether an alternative answer got the right number of marks, should in my view be answered by the supervisor / instructor -- any dispute over such a matter will in the end involve him/her anyway.

And as a student, If I have reason to believe that the marker is not too familiar with the material, I would go straight to the instructor.