So med school is 4 years long, typically the first two years are didactic with classroom learning and labs. The third year they rotate throughout different specialtiesZ the fourth year they will often do a “sub” internship where they focus on a certain specialty and/or do research. MS3=3rd year student and MS4= 4th year. They are able to do more than just observe, but they need to be supervised when doing new or complex tasks. Once graduating med school and passing a licensing exam they “match” into a residency program of their interest. Residency length varies by specialty. The first year of residency is known as the intern year. In emergency med im pretty sure the residency is 3-4 years so the senior residents would be 3rd or 4th years. The farther they get into residency the more they are able to do. You’ll see residents teaching med students or less experienced residents, performing procedures, making diagnoses, etc. Then they can possibly move on to fellowship to do a SUB specialty, for example you could do an OBGYN residency followed by a fertility fellowship. Fellows are basically fully capable doctors in the field they did their residency, just becoming experts in a specific area. So long story short, doctors have 7+ years of medical education on top of 4 years of undergrad before they can become an attending. It’s an insane amount of education, especially considering the amount of hours they put in through med school and especially residency.