Each school has its own way of doing a "committee letter". Some schools gather your letters of recommendation and a single person, or perhaps a pair, write a formulaic letter discussing your background, academic performance, extracurricular activities, and your personal characteristics quoting liberally from the letters you have asked to have provided on your behalf. These letters are usually authored by a pre-med advisor or someone in a pre-med office.
Some committees ask for your application including grades, experiences and personal statement and they interview you. The letter then highlights what is in your application.
Finally, some schools merely serve as letter services with, perhaps, a cover form that asks the letter writer to categorize you in things like maturity, written communication skills, verbal communications, creativity, integrity, etc.