Certain schools provide evaluation sheets. LSU and Louisville are examples. Letters can be sent as individuals are composed into one. When composed into one, your healthcare advisor will take exerpts and quotes from all your references and make one complete reference letter. I was able to glance at mine during an interview, just to see how it was organized (I just snuck a peek). If your advisor knows you well, they try and use the reference letter to paint a picture of who they think you are as a student and person. So they reference the other letters when writing about you.
Of course, not all letters are done this way. Just an example