The following question is being considered to be a duplicate of the second.
How do I become a journal reviewer?
How do you earn opportunites to review journals or conference papers?
I was basing the first question on the second. I'm not doubting the opportunities to review papers as a grad student or a post-doc. However, I'm more curious about how one becomes the primary reviewer vs. being a person who helps out with the review. I'm curious about how to separate the two questions or potentially merge them.