Fellowships

Do fellowships follow the traditional route that medical schools and residencies follow? i.e. What medical school you attend depends on college grades, MCAT, ECs, etc. > What residency you get depends on medical school grades, USMLE, ECs, letters, etc. > So what criteria are used to determine fellowships? Is it strictly based on residency performance, letters, etc. or do they go back further (medical school) to decide on fellows?

Thank you.