How does score normalization among different graders work? In my university, in a class with several hundred students, there are teaching assistants who are assigned a small fraction of the students for assignments which they grade. I have heard that they normalize scores among these teaching assistants to make it fair. How does that work?
There are many possible systems and so it varies with each situation. One is to use some statistical analysis, which can actually be unfair. It needs to make an assumption, probably that each group of students is a fair sample, which it isn't. 

Another is to have someone look at a sample question from different graders and make adjustments. 

One method that I've seen used on final exams is to have each grader grade one question from everyone in the course. So that each student is graded by several people. 

Another possible method is to have each paper graded twice. 

You can probably come up with more possibilities. 

Some of these might be combined with advising the graders who seem to be too easy or too hard. 

Most of them probably start out with grading rubrics for each question that graders are supposed to adhere to. 

For your own situation, you need to inquire locally. There may be someone in the department who is responsible for all TAs and who coordinates these things. 

I'll also note that doing nothing at all to normalize scores is also a possibility. In fact, when different professors teach small sections of the same course (no common lectures), it is likely to be the case.